35
Monetary Policy, Commodity Prices, and Misdiagnosis Risk Andrew J. Filardo, a Marco J. Lombardi, a Carlos Montoro, b and Massimo Minesso Ferrari c a Bank for International Settlements b Central Reserve Bank of Peru and Professor of CENTRUM Cat´ olica, Pontificia Universitad Cat´ olica del Per´ u c European Central Bank How should monetary policy respond to commodity prices when the underlying drivers are difficult to diagnose accu- rately? If monetary authorities misdiagnose commodity price swings as being driven primarily by external supply shocks when they are in fact driven by global demand shocks, the conventional wisdom—to look through the first-round effects of commodity price fluctuations—may no longer be sound policy advice. To analyze this question, we employ the multi-economy DSGE model of Nakov and Pescatori (2010), which splits the global economy into commodity-exporting and non- commodity-exporting economies. In an otherwise conventional DSGE setup, commodity prices are modeled as changing endogenously with global supply and demand developments, including global monetary policy conditions. This framework allows us to explore the implications of monetary policy deci- sions when there is a risk of misdiagnosing the drivers of com- modity prices. We are grateful to the editor and a referee for their helpful and construc- tive comments. We thank conference participants at the Central Bank of Brazil’s XVII Annual Inflation Targeting Seminar, CEMLA’s XX Annual Meeting, the 2017 Central Bank Research Association (CEBRA) Annual Meeting, the 2019 Annual International Journal of Central Banking Research Conference, and the Second Macroeconomic Modelling and Model Comparison Network at Stanford University. We also thank seminar participants at the Bank for International Set- tlements, the Bank of Canada, and the Bank of Japan for their comments. The views expressed here are those of the authors and do not necessarily reflect those of the Bank for International Settlements, the Central Reserve Bank of Peru, or the European Central Bank. 45

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Page 1: Monetary Policy, Commodity Prices, and Misdiagnosis Risk · Monetary Policy, Commodity Prices, and ... (2013) and Filardo and Lombardi (2014) report evidence that loose monetary policy

Monetary Policy, Commodity Prices, andMisdiagnosis Risk∗

Andrew J. Filardo,a Marco J. Lombardi,a Carlos Montoro,b

and Massimo Minesso FerraricaBank for International Settlements

bCentral Reserve Bank of Peru and Professor of CENTRUM Catolica,Pontificia Universitad Catolica del Peru

cEuropean Central Bank

How should monetary policy respond to commodity priceswhen the underlying drivers are difficult to diagnose accu-rately? If monetary authorities misdiagnose commodity priceswings as being driven primarily by external supply shockswhen they are in fact driven by global demand shocks, theconventional wisdom—to look through the first-round effects ofcommodity price fluctuations—may no longer be sound policyadvice.

To analyze this question, we employ the multi-economyDSGE model of Nakov and Pescatori (2010), which splitsthe global economy into commodity-exporting and non-commodity-exporting economies. In an otherwise conventionalDSGE setup, commodity prices are modeled as changingendogenously with global supply and demand developments,including global monetary policy conditions. This frameworkallows us to explore the implications of monetary policy deci-sions when there is a risk of misdiagnosing the drivers of com-modity prices.

∗We are grateful to the editor and a referee for their helpful and construc-tive comments. We thank conference participants at the Central Bank of Brazil’sXVII Annual Inflation Targeting Seminar, CEMLA’s XX Annual Meeting, the2017 Central Bank Research Association (CEBRA) Annual Meeting, the 2019Annual International Journal of Central Banking Research Conference, and theSecond Macroeconomic Modelling and Model Comparison Network at StanfordUniversity. We also thank seminar participants at the Bank for International Set-tlements, the Bank of Canada, and the Bank of Japan for their comments. Theviews expressed here are those of the authors and do not necessarily reflect thoseof the Bank for International Settlements, the Central Reserve Bank of Peru, orthe European Central Bank.

45

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46 International Journal of Central Banking March 2020

We first confirm that monetary authorities deliver bettereconomic performance when they are able to accurately iden-tify the global nature of the shocks, i.e., global supply anddemand shocks, driving commodity prices. Moreover, we showthat when it is difficult to identify these shocks, monetaryauthorities can minimize some of the adverse feedbacks frommisdiagnoses by targeting core inflation. Finally, we highlightthe implications of misdiagnosis risk in the case where themonetary authority misinterprets supply-driven increases incommodity prices as demand driven; the contraction in bothoutput and core inflation is larger than in the case of an accu-rate diagnosis.

In light of recent empirical studies documenting the sig-nificance of global demand in driving commodity prices, thesefindings call for giving greater prominence to global factors indomestic monetary policymaking and highlight potential gainsfrom focusing on accurate diagnoses of domestic and globalsources of shocks.

JEL Codes: E52, E61.

1. Introduction

Over the past decade, global commodity prices have experiencedwide swings, reaching historically high levels in the run-up to theGreat Recession before plummeting as the global economy collapsed.Prices subsequently rebounded with the global economic recoverybut then fell again amid significant policy concerns. While challeng-ing, this type of volatility is not a new environment for policymakers.Even though most commodity prices remained broadly stable dur-ing the so-called Great Moderation, they were quite volatile in the1970s amid geopolitical tensions that pushed oil price volatility tothen-unprecedented levels.

It was the experience of the 1970s that forged the conventionalwisdom about how monetary authorities should respond to com-modity price fluctuations. Commodity price fluctuations were seenlargely as the result of exogenous supply shocks; in such an environ-ment, the conventional wisdom that emerged was that, when facingsuch swings, monetary authorities should look through the first-round price effects and only respond to the second-round effects

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Vol. 16 No. 2 Monetary Policy, Commodity Prices, Misdiagnosis 47

on wage and inflation expectations. In practice, this suggested amonetary policy focus on core inflation.

Views about the drivers of global commodity price swings havebeen evolving, especially in recent years, as a growing body ofstatistical evidence points to a new interpretation of commodityprice swings. Kilian (2009), for example, finds evidence that oilprice fluctuations have been increasingly influenced by demandfrom commodity-hungry emerging market economies (EMEs). Themost recent literature has not challenged this view: Kilian andBaumeister (2016) argue that the oil price decline in 2015 shouldalso be ascribed to a slowdown in global economic activity; andStuermer (2017) and Fueki et al. (2018) emphasize the role ofdemand shocks in a historical perspective. In a similar vein, Suss-man and Zohar (2016) report that commodity price fluctuationscan be taken as a proxy for global demand. In a broader con-text, Filardo and Lombardi (2014) note the growing prominenceof these global demand shifters for EME inflation dynamics. Thisevidence raises doubts about the relevance of the conventionalwisdom.

The prominence of endogenous commodity price swings hasimportant implications for monetary policy, given its central rolein influencing aggregate demand. The relationship between mone-tary policy decisions and endogenous commodity prices implies animportant two-way link. Monetary policy decisions influence aggre-gate demand and hence commodity prices. Indeed, Anzuini, Lom-bardi, and Pagano (2013) and Filardo and Lombardi (2014) reportevidence that loose monetary policy has had an effect on commodityprices via the global demand channel.1 At the same time, commod-ity price swings influence price stability and hence monetary policydecisions.

Given the global nature of commodity prices, misdiagnosis riskis particularly high in a world of many central banks with purelydomestic monetary policy mandates. Individual countries may thinkthat because they are sufficiently small, they can reasonably ignorethe effect of their own policy decisions on the rest of the world,

1There is evidence that U.S. monetary policy plays a special role. Akram(2009) finds that lower interest rates in the United States boost commodity pricesvia an exchange rate channel.

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and hence on commodity prices. This would be the case if alleconomies were hit by uncorrelated idiosyncratic shocks. However,global shocks imply that central banks are likely to respond in acorrelated way which would endogenously feed back on commod-ity prices. A failure to internalize the endogenous feedbacks couldcontribute to economic and financial stability concerns (Caruana,Filardo, and Hofmann 2014; Rajan 2015).

From a modeling perspective, this discussion suggests the impor-tance of developing monetary policy models with endogenouslydetermined commodity prices and monetary authorities that aresubject to misdiagnosis risk. To date, the bulk of the theoreticalliterature has stayed clear of models with endogenous commodityprices (see, e.g., Leduc and Sill 2004, Carlstrom and Fuerst 2006,Montoro 2012, Natal 2012, and Catao and Chang 2015). Moreover,this literature has generally focused on how a monetary authorityshould respond to exogenous movements in oil prices, e.g., whether itis optimal to target core or headline inflation and whether commod-ity price movements have far-reaching implications for the tradeoffbetween stabilizing output and controlling inflation. For example,Blanchard and Galı (2010) have gone so far as to argue that anincrease in commodity prices driven by foreign demand can stillbe treated by a domestic monetary authority as an external sup-ply shock. Such a conclusion is less tenable in models of endoge-nous commodity prices and correlated monetary policy reactionfunctions.

Various theoretical papers have addressed the endogeneity ofcommodity prices in small-scale dynamic stochastic general equilib-rium (DSGE) models (e.g., Backus and Crucini 2000; Bodenstein,Erceg, and Guerrieri 2008; and Nakov and Nuno 2013). How-ever, these models have generally ignored monetary policy, focus-ing instead on oil price determination and the frictions affectingit. Nakov and Pescatori (2010) is an early attempt to characterizemonetary policy tradeoffs in a DSGE model in which oil prices aredetermined endogenously. Another important contribution to thisliterature is Bodenstein, Guerrieri, and Kilian (2012), who highlight,as we do, the benefits of identifying the nature of the shocks hittingthe economy.

Our model extends this class of models by considering the pol-icy challenges facing a monetary authority when it tries to infer the

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Vol. 16 No. 2 Monetary Policy, Commodity Prices, Misdiagnosis 49

source of commodity price shocks.2 Namely, there is a risk that amonetary authority may misdiagnose a commodity price swing asbeing driven by an external supply shock when it is, in fact, drivenby an endogenous global demand shock, and vice versa. In our model,the commodity price is endogenously determined in equilibrium bythe interplay of global demand and commodity supply from twotypes of commodity-exporting economies—one competitive and onemonopolistic. In this setting, the optimal monetary policy responseto commodity price swings depends on the perception of the under-lying drivers of the swings. Unable to fully know the nature of thedrivers, the monetary authority infers them via signal extraction,thereby opening up the possibility of systematic misdiagnoses.

The modeling exercise delivers several policy-relevant implica-tions. First, it is important to distinguish between global demandand supply shocks when responding to commodity prices. If it is pos-sible to accurately diagnose the source of a shock, our model findsthat the best response to demand shocks is to lean against themfully (a result consistent with a standard New Keynesian closed-economy model). The best response to commodity supply shocks(i.e., a decrease in commodity prices) is to look through them.

Second, the conventional wisdom of looking through the first-round effects of commodity price swings is not always optimal. In ourmodel, this result arises because our model breaks the “divine coin-cidence” between inflation and output gap stabilization (e.g., Blan-chard and Galı 2007), which is a standard feature of DSGE modelswith exogenous commodity prices. The breaking of the divine coin-cidence comes, in part, from the assumption of a monopolisticallycompetitive commodity exporter, and in part from the imperfectinformation environment.

Third, misdiagnosis risk matters. In the case where the monetaryauthority misinterprets supply-driven increases in commodity pricesas demand driven, the contraction in both output and core inflationis larger than in the case of an accurate diagnosis. This indicatesanother reason for the breakdown of the divine coincidence in thismodel (even if the dominant exporter acts as a price taker). This

2See Filardo and Lombardi (2014) for a discussion of commodity price misdi-agnosis risks in the context of Asian EMEs.

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result underscores the potential benefits of trying to correctly diag-nose the sources of commodity price swings when setting monetarypolicy. For example, a monetary authority that misdiagnoses globaldemand shocks as external supply shocks amplifies cyclical fluctua-tions (including commodity prices) and, as a result, destabilizes theeconomy.

2. Outline of the Baseline Model

We present a global monetary policy model in which commodityprices are determined endogenously, in the spirit of Nakov andPescatori (2010, hereafter NP). The global economy is split intocommodity-importing countries and commodity-exporting ones.

The commodity-importing countries are treated as one represen-tative economy. It imports the commodity both for consumptionand as an input in production of final goods and services. The firmsproduce final goods and services in a monopolistically competitiveway in the face of nominal rigidities.

The commodity-exporting economies comprise a dominantcommodity-exporting economy and a fringe of smaller competitiveexporters. The dominant commodity-exporting economy has mar-ket power and sets prices above marginal cost. The fringe of smallexporting countries operates competitively, taking the global com-modity price as given. Consumers in these commodity-exportingcountries buy final goods and services from the commodity-importing economy.3

The role of the central bank is to set monetary policy a la Tay-lor (1993). We extend the NP model by considering two types ofuncertainty that the central bank faces. The first is real-time datauncertainty. Because inflation and output are typically only observedwith a lag, we model the central bank as reacting to expected infla-tion and the expected output gap. The central bank can observe and

3Note that cross-border financial autarky is assumed, implying that currentaccounts are balanced in each period. Also, trade is assumed to be carried outin a common global currency, suppressing potential tradeoffs from exchange ratedynamics. These assumptions streamline the analysis and allow us to highlightthe key implications of misdiagnosis risk which would be more complex in a richermodel.

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respond to commodity prices in real time. Relaxing the perfect infor-mation assumption opens up one way, within this class of models,for us to explore the consequences of central bank miscues in set-ting the policy rate. Second, and much less frequently addressed inthe literature, is uncertainty about whether the commodity price isdriven by demand or supply shocks. We assume that central bankscannot directly observe the source of commodity price shocks, butpolicymakers use available data to infer them. We explore the infer-ential challenges and implications that arise from the misdiagnosis ofshocks, thereby shedding further light on the practical use of DSGEmodels.

The rest of this section sketches out the structure of the model.In addition to the two key points mentioned above, we also deviatefrom the Nakov and Pescatori (2010) setup in three other respects:(i) we introduce the commodity good into the households’ utilityfunction, which allows us to consider nontrivial policy responses todifferences between headline and core inflation; (ii) we interpret thecommodity as a broad basket of commodities rather than focusingnarrowly on oil;4 and (iii) we solve the Nash (instead of the Ram-sey) problem for the dominant producer so as to reflect realisticinformation constraints facing producers.5

2.1 Representative Households and Firms in theCommodity-Importing Economy

Imported commodities play two roles in this economy. For house-holds, commodities enter the consumption basket; as mentionedabove, we make this dependence explicit to highlight the mone-tary policy implications of headline and core inflation. For firmsin the commodity-importing economy, commodities are used in theproduction of final goods and services.

4This is consistent with the view that commodity markets as a whole remainan important driver of short-run inflation dynamics and often reflect cartel-likebehavior for metals (and minerals) and anti-competitive behavior for foodstuffs(see Organisation for Economic Co-operation and Development 2012).

5The computer code for the model is available at http://www.macromodelbase.com/.

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2.1.1 Representative Households

The representative household has a utility function over consump-tion (Ct) and labor (Lt),

Ut0 = Et0

∞∑t=t0

βt−t0exp (et)[ln (Ct) − L1+v

t

1 + v

], (1)

where et is a preference shock v and is the inverse of the labor supplyelasticity. Consumption is defined as a Cobb-Douglas aggregator offinal goods and services, CY, t, and the commodity, MC,t:

Ct = (CY,t)1−γ (MC,t)

γ. (2)

CY, t is a Dixit-Stiglitz aggregate of a continuum of differ-entiated goods and services, CY, t(z), of the form CY,t =[∫ 1

0 CY,t (z)ε−1

ε dz] ε

ε−1.

The household maximizes its intertemporal utility subject to theperiod budget constraint,

Ct =Wt Lt

Pt+

Bt−1

Pt− 1

Rt

Bt

Pt+

Γt

Pt+

Tt

Pt, (3)

where Wt is the nominal wage, Pt the price of the consumption good,Bt the end-of-period nominal bond holdings, Rt the riskless nomi-nal gross interest rate, Γt the share of the representative household’snominal profits, and Tt the net transfers from the government.

2.1.2 Firms

Monopolistically competitive firms in the commodity-importingeconomy produce final goods and services using a Cobb-Douglastechnology:

Yt (z) = AtLt (z)1−αMY,t (z)α

, (4)

where MY,t is the commodity input and α denotes the commodityshare in production. The firms’ cost-minimization problem implies

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Vol. 16 No. 2 Monetary Policy, Commodity Prices, Misdiagnosis 53

an expression for the real marginal cost, given the real commoditymarket price, Qt ≡ PM,t/Pt:

MCt (z) =(

Wt

Pt

)1−α

Qαt

/ [At (1 − α)1−α

αα].6 (5)

These first-order conditions imply the demand equations for thecommodity:

MY,t (z) = αMCt

QtYt (z) . (6)

Under Calvo pricing, and following Benigno and Woodford(2005), the implied nonlinear Phillips curve for final goods andservices—i.e., core inflation (ΠY,t)—is

θΠY,t = 1 − (1 − θ)(

Nt

Dt

)1−ε

, (7)

where Nt/Dt is the equilibrium relative price, with Dt =Yt/Ct + θβEt

[(ΠY,t+1)

ε−1 Dt−1

]and Nt = μYtMCt/Ct +

θβEt [(ΠY,t+1)ε Nt−1].

2.1.3 Aggregation

Aggregating firms’ demand for labor and the commodity yields aset of equations with which to solve the model. Even though laborand commodity demand differ across firms due to Calvo staggeredpricing, the aggregate-demand equations resemble those of the indi-vidual firm’s demand equations except for an adjustment capturingthe effect of price dispersion. Higher inflation increases the disper-sion of prices, and this increased price dispersion boosts labor andcommodity demands for a given level of output.

6Note that all firms face the same real marginal costs given the constant-returns-to-scale technology and competitive factor markets. In this model, thereal commodity price is proportional to the inverse of the importing economy’sterms of trade.

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2.2 Representative Households and Firms in theCommodity-Exporting Economies

The commodity market is modeled as comprising a dominant com-modity exporter and a fringe group of competitive commodityexporters. The dominant exporter produces monopolistically, andthe fringe produces competitively. For completeness, the householdsin these economies face conventional decision problems.

2.2.1 Dominant Commodity Exporter

In each period, the dominant exporter chooses its supply to max-imize profits, taking global demand as given and internalizing theresponse of the competitive fringe. The dominant exporting economyproduces the commodity according to the technology:

Mt = ZtI∗,Dt , (8)

where Zt is an exogenous productivity shifter and I∗,Dt is the inter-

mediate input (bought from the commodity-importing economy).Productivity evolves exogenously according to

lnZt = (1 − ρz) ln Z + ρz lnZt−1 + εzt , (9)

where εzt ∼ i.i.d.N

(0, σ2

z

). Shocks to Zt can then be interpreted as

global commodity supply shocks.The household utility function depends only on consumption of

final goods and services:

U∗,Dt0 = Et0

∞∑t=t0

βt−t0 ln(C∗,D

t

)(10)

subject to the period budget constraint, PY,tC∗,Dt = Γ∗,D

t , whichequates consumption expenditure to dividends from commodity pro-duction, Γ∗,D

t .

2.2.2 Fringe of Competitive Commodity Exporters

The fringe exporters comprise a continuum of atomistic firmsindexed by j ∈ [0, Ωt]. Each produces a quantity Xt(j) of the com-modity according to technology of the form

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Vol. 16 No. 2 Monetary Policy, Commodity Prices, Misdiagnosis 55

Xt (j) = ξ (j) ZtI∗,Ft (j) , subject to capacity constraint

Xt (j) ∈[0, X

], (11)

where [ξ (j) Zt]−1 is the marginal cost of economy j defined by an

idiosyncratic shock and global supply shock. Input I∗,Ft (j) is an

intermediate input used in commodity production and is boughtfrom the commodity-importing economy. Each firm maximizes prof-its, taking as given the international real price of the commodity,Qt,

max QtXt (j) −PY,t

PtXt (j)

ξ (j) Zts.t. Xt (j) ∈

[0, X

]. (12)

The resulting supply by the fringe of competitive exporters is Xt =ΩtZtQt.

Like the setup for the dominant exporting economy, the house-hold utility function in the fringe economies depends only on theaggregate consumption of final goods and services:

U∗,Ft0 = Et0

∞∑t=t0

βt−t0 ln(C∗,F

t

), (13)

subject to the following period budget constraint, PY,tC∗,Ft = Γ∗,F

t ,which equates consumption expenditures to dividends earned by thefringe commodity exporters, Γ∗,F

t .

2.2.3 Aggregate Production of the Commodity

In the case of a perfectly competitive fringe market of commod-ity exporters, the equilibrium commodity price is equal to marginalcosts:

QPCt = Z−1

t , (14)

and the quantity produced is given by the global demand at thatprice. Having market power, the dominant commodity exporterdetermines its supply by internalizing the optimal response of the

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56 International Journal of Central Banking March 2020

fringe and the global demand function for the commodity. The dom-inant producer’s objective function is

max{Mt}∞

t0

Et0

∞∑t=t0

βt−t0 ln(Q

1/1−γt Mt − Mt/Zt

)(15)

given commodity demand and supply from fringe suppliers, Mt =MC,t + MY,t − Xt.7

The first-order condition of this problem determines the com-modity price (in real terms):

Qt = ΨtZ−1t , (16)

where Ψt ≡ 1/ (1 − ηt) is the commodity market markup andηt = Mt/ (Mt + 2Xt) is the elasticity of substitution of the globaldemand for the commodity (in absolute value). Accordingly, thecommodity price is a markup over marginal cost, the latter beinginfluenced by commodity supply shocks, firm-specific supply shocks,and shifts in gt.8

2.2.4 Market Clearing

Under the assumption that all relevant markets clear, there is awell-defined aggregate demand for final goods and services equal toaggregate supply,

Yt = CY,t + C∗,Dt + I∗,D

t + +C∗,Ft + +I∗,F

t + gt, (17)

7Note that the dominant exporter takes as given the macroeconomic variablesof the fringe producers, e.g., (C, MC, Y, Ω, Δ), and internalizes the fringe’s reac-tion, but not the feedbacks of its actions on the macroeconomic performanceof the commodity-importing economy. Nakov and Pescatori (2010) analyze thiscase in which the dominant exporter completely internalizes its actions on theimporting country. Technically, they solve the Ramsey problem of the dominantexporter, which may be useful in capturing longer-run supply behavior in thedata. Our Nash solution is more restrictive but may be seen as more reasonablewhen considering a central bank’s short-run tradeoffs in a world with incompleteinformation. Further details on the dominant exporter’s problem are provided inappendix B.

8In addition, the markup Ψt ≡ 1/ (1 − ηt) is an increasing function of thedominant commodity exporter’s market share relative to that of the competitivefringe of commodity exporters. The limiting case is when Mt → 0 correspondsto perfect competition while Xt → 0 is the case of a single monopolist.

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which includes final goods consumption in the commodity-importingeconomy and the aggregate consumption and intermediate goodsdemanded by the dominant and the competitive fringe of thecommodity-exporting countries, respectively (superscript D denotesthe dominant commodity-exporting economy and F the competitivefringe).9 Finally, gt

10 is an aggregate demand shock, which capturesa positive shift in the demand for the final good produced by thecommodity-importing country. Note that the monetary authoritycannot fully offset the effect of this shock in the same way as itcould with a markup shock.

2.3 Characterizing Monetary Policy

Monetary policy in the commodity-importing country is modeledas a linear Taylor-type rule informed by deviations from model-consistent benchmarks for output, inflation, and the interest rate.This section defines the benchmarks and analyzes the implicationsof alternative policy rules.

2.3.1 Optimal Benchmarks

Benchmark output gaps can be derived by substituting the equationsfor labor demand, labor supply, aggregate demand, and commoditydemand for production into the aggregate production function. Thelog-level of output in terms of marginal costs, the dispersion of prices,productivity, and the real commodity price is11

yt =1

1 − αat +

α

1 − α(mct − qt) +

11 + v

Υ(

mct +γ

1 − γqt

).

(18)

9The structural parameters in the model are calibrated to be in line with thosefound in the literature (see table A.1 in appendix A). Fiscal policy is modeled asa linear rule: Tt = τPY,tYt.

10gt is modeled as an AR(1) process with zero mean, i.e., gt = ρggt−1 + εgt .

11The level of output is Yt =(

AtΔt

)1/(1−α) [(1−α)MCtΔt

(Qt)−γ/(1−γ)−αMCtΔt

]1/(1+v)

[αMCtΔt

Qt

]α/(1−α)and Υ ≡

[1 − α

μQγ/(1−γ)

]−1≥ 1. A full derivation is provided

in appendix C.

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58 International Journal of Central Banking March 2020

The level of natural output, ynt , is defined as the level of output

consistent with a flexible-price equilibrium. In this case, the mar-ginal cost is constant, MCt = μ−1, and there is an absence of pricedispersion, Δt = 1. In log-linear terms, the level of natural output,yn

t , is of the form

ynt =

α

1 − αat −

1 − α− 1

1 + vγ

1 − γΥ

)qt. (19)

As shown in equation (19), commodity price fluctuations havetwo opposing effects on the level of natural output. From the side ofproduction (i.e., the first term in the parentheses), an increase in thecommodity price has a qualitative effect similar to that of a negativeproductivity shock; it reduces the level of natural output. From theside of consumption (i.e., the second term in the parentheses), anincrease in the commodity price increases the level of natural out-put. The latter term reflects an increase in labor due to a negativeincome effect from a higher commodity price.

The natural output gap, ynt =

1−α − 11−v Υ

]mct, measures the

difference between the actual and the natural level of output. Thisimplies that responding to the natural output gap is equivalent toresponding to real marginal costs, up to a scale factor.

Similarly, the log-level of efficient output, yet , is defined with

respect to the efficient allocation, i.e., flexible prices and no monop-olistic distortions in the commodity market or in the final goodsmarket (which implies that Qe

t = Z−1t and μe = 1):12

yet =

α

1 − αat −

1 − α− 1

1 + vγ

1 − γΥe

)zt. (20)

A key difference is that commodity markup shocks do not affectthe level of efficient output: such output is instead affected only byfluctuations associated with supply shocks in the commodity market.As a consequence, a demand-driven increase in the commodity price

12With Υe ≡[1 − αZ−γ/(1−γ)

]−1. The relationship between Υe and Υ depends

on the extent of monopolistic distortions. Υe and Υ are equal only if both marketsare perfectly competitive or if the commodity is not used for production (that is,α = 0).

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would leave the benchmark efficient output gap unchanged. How-ever, a negative commodity supply shock would decrease both thenatural and efficient output levels, albeit by different amounts.13 Theefficient output gap, ye

t , which is defined as the difference betweenactual output and the efficient level of output, is of the form14

yet = yn

t −(

α

1 − α− 1

1 + vγ

1 − γΥ

)Ψt − 1

1 + vγ

1 − γ(Υ − Υe)Zt,

(21)

where this is the welfare-relevant output gap and is equal to the nat-ural output gap plus a term that depends on the commodity pricemarkup and the commodity supply shock.

2.3.2 Breaking Down the Divine Coincidence

Both core inflation and headline inflation are determined by the nat-ural output gap, expected inflation, and commodity price changes.Expressed in log-linear terms, the equations for core inflation andheadline inflation are, respectively,

πY,t = κyynt + EtπY,t+1 and (22)

πt = πY,t +γ

1 − γΔqt. (23)

Equation (22) describes the determinants of core inflation, and equa-tion (23) describes headline inflation written in the form of a Phillipscurve for aggregate final goods with yn

t the natural output gap. Sta-bilization of the natural output gap is equivalent to stabilization ofcore inflation. And, in that case, headline inflation would vary pro-portionally with changes in real commodity prices. Equation (22)can be written in terms of the efficient output gap:

πY,t = κyyet + EtπY,t+1 + ut, (24)

13The level of efficient output contracts less than the level of natural outputin response to a negative commodity supply shock because the commodity pricemarkup partially offsets the effects of supply shocks on the commodity price.

14Ψt and zt are Ψt and Zt expressed in log-deviations from the steady state;

Υe ≡[1 − αZ−γ/(1−γ)

]−1.

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where ut is an endogenous cost-push shock, which is a function ofboth Ψt and zt. In this model, the divine coincidence featured inmodels with exogenous commodity prices is broken. It is no longerpossible to simultaneously stabilize core inflation and the welfare-relevant output gap. The tradeoff arises from the effect of commod-ity price fluctuations on the level of efficient output. An increasein commodity price markups generates a positive cost-push shock,which puts upward pressure on core inflation but lowers the efficientoutput gap.

3. Monetary Policy Responses to Commodity Prices

With the model outlined in section 2 (and calibrated as reported inappendix A), we now explore the performance of alternative mon-etary policy rules assuming different types of imperfect informa-tion. This allows us to focus on both the possibility that centralbanks may misdiagnose the nature of the shocks hitting an economyand the associated implications. The imperfect information settingsalso highlight the absence of the divine-coincidence property: mon-etary policy cannot perfectly stabilize inflation and the output gapunless the central bank is able to accurately identify the nature ofunderlying shocks.15

3.1 Monetary Policy Responses under Conventional DataUncertainty

The baseline policy rule assumes that the monetary authorityresponds to expectations of core inflation and the efficient outputgap. It is a log-linear Taylor-type rule:

rt = Et|t−1 [ret + ϕcoreπY,t + ϕy (yt − ye

t )] + ϕcomΔqt, (25)

15Consistent with the Nakov and Pescatori (2010) model, we focus on theimplications of uncertainty for monetary policy tradeoffs only in the commodity-importing economy by assuming nominal rigidities are absent in the commodity-exporting countries.

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Vol. 16 No. 2 Monetary Policy, Commodity Prices, Misdiagnosis 61

Figure 1. Responses to a Positive Aggregate-DemandShock (εg

t )

Notes: Impulse responses to an aggregate-demand shock using policy rulert = Et|t−1(re

t + ϕcoreπY,t + ϕy yet ) + ϕcomΔqt. The shock is calibrated so as

to generate a 1 percent increase in commodity prices.

which includes a benchmark interest rate (ret ), core inflation (πY,t) ,

and output gap yet = (yt − ye

t ). We assume that the monetaryauthority responds to expected inflation and output gap, whereasthe change in the commodity price (Δqt) is available in real time.

The performance of policy rule (25) is graphically assessed usingimpulse responses to demand and supply shocks.16 Figure 1 plotsthe responses to a positive aggregate demand shock, defined as anexogenous shift in aggregate demand through gt in equation (17).Figure 2 plots the response to a negative supply shock, defined as

16The coefficients of the policy rule are set to ϕcore = 1.5, ϕy = 0.5, andϕcom = 0.05.

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Figure 2. Responses to a Negative Supply Shock (εzt )

Notes: Impulse responses to a (negative) supply shock. Other details are infigure 1.

an exogenous shift in Zt in equation (9). Both shocks are calibratedto produce, on impact, a 1 percent increase in the commodity price.

Following a demand shock (figure 1), the commodity price rises(top-center panel). The demand shock also drives up output (butnot the level of the efficient output) and the output gap. Both coreand headline inflation initially rise. With the efficient interest rateunchanged, the policy rule calls for an increase in the policy rate(top-right panel). For roughly six quarters, the interest rate and out-put gap remain elevated. Note that the intuitively plausible shapesof the response underscore the reasonableness of our calibration forpolicy analysis.

A negative supply shock (figure 2) drives up the commodityprice, but the policy rate response as well as the macroeconomicones are modest. This constellation of responses confirms that the

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Table 1. Expected Welfare Loss for Alternative EfficientPolicy Rules, Percent of Steady-State Consumption

(× E – 4)

Model

(1) (2) (3) (4) (5)

Welfare Loss → –3.80 –6.05 –5.91 –4.98 –5.27

Policy Rule Specificationsfor Each Model

ϕcore 2.0 — 1.5 1.5 2.0ϕhead — 2.0 — — —ϕy — — 0.5 0.5 1.0ϕcom — — — 0.05 0.1

Notes: The unconditional expected welfare, EW t, is assessed using a second-ordersolution of the model, where Wt = U(Ct, Lt, et) + βEtWt. The welfare cost in termsof steady-state consumption is equivalent to {exp((1 − β)(EWt − W ))} × 100. TheTaylor-type monetary policy rule is specified as rt = Et|t−1[re

t +ϕcoreπY,t+ϕheadπt+ϕy (yt − ye

t )] + ϕcomΔqt.

baseline model (with its imperfect information assumption) deliversthe conventional wisdom that monetary authorities should essen-tially look through the first-round effects of supply shocks.

Table 1 provides an alternative metric of model performancebased on the model’s welfare criterion under different policy rules.The first column corresponds to a rule with a response only toexpected core inflation; it achieves the lowest (expected welfare) loss.Columns 2–5 correspond to alternative policy rules. The rule in col-umn 2 considers a response only to expected headline inflation; thisproduces the worst outcome, indicating the superiority of targetingcore versus headline inflation in our model. Column 3 introduces anadditional response to the expected output gap, and columns 4 and5 add a direct response to the commodity price change.17

17One drawback of evaluating expected welfare losses with simple linear rulesis that the results may be sensitive to the variances of the shocks. We have con-ducted a battery of tests to confirm that our findings are robust to a range ofreasonable variances.

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Figure 3. Response to a Positive Aggregate-DemandShock under Full Information

Notes: Impulse responses to an aggregate-demand shock using the baseline pol-icy rule of figure 1 and the full-information rule rt = re

t +ϕcoreπY,t+ϕy (yt − yet )+

ϕcomΔqt. The shock is calibrated so as to generate a 1 percent increase incommodity prices under the baseline policy rule.

While results suggest that responding to commodity prices canimprove performance over responding to headline inflation, theyleave open questions such as whether monetary authorities canachieve even better performance by inferring the nature of the shocksdriving commodity price developments. Data uncertainty indeedinduces substantial changes in the reaction to shocks. To illustratethis, we compare the baseline results with those under full infor-mation, i.e., when the monetary authority is able to observe withcertainty macrovariables at each point in time. Figures 3 and 4 com-pare the reaction of the system to demand and shocks with thebaseline. The overall picture is that first-round effects generated byuncertainty are already sizable. Looking at the aggregate-demandshock (figure 3) highlights that under full information the monetary

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Figure 4. Response to a Negative Supply Shock underFull Information

Notes: Impulse responses to a supply shock using the baseline policy rule offigure 1 and the full-information rule rt = re

t +ϕcoreπY,t+ϕy (yt − yet )+ϕcomΔqt.

The shock is calibrated so as to generate a 1 percent increase in commodity pricesunder the baseline policy rule.

authority increases the interest rate more aggressively comparedwith the baseline. This is no surprise, given that the monetaryauthority is fully aware of the effects of the shock on output. Bydoing so, the monetary authority not only dampens cyclical fluctua-tions but also moderates the increase in commodity prices and hencein inflation. Also, under a supply shock (figure 4) full informationhelps the monetary authority stabilize output, although in this casedifferences are less striking.

3.2 Monetary Policy Responses under Misdiagnosis Risk

In addition to uncertainty about current output and inflation,when setting policy rates monetary authorities do not have precise

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knowledge of the underlying nature of the shocks affecting com-modity prices. This section considers the challenges of inferring thenature of the shocks and implications arising from possible misdiag-noses, i.e., the risk that a monetary authority may misinterpret thesource of the shock driving changes in commodity prices.

We cast the problem of shock identification in a classical signal-extraction problem.18 Starting with the assumption that the mone-tary authority does not observe supply and demand shocks (εz

t andεg

t ), it can infer them from available observations. Consider a simplelinear model for commodity prices:

qt = −zt + ψt = H ′tξt, (26)

where zt and ψt are the supply and the demand shock in the centralbanks’ signal-extraction model, Ht = [−1 1], and ξt = [zt ψt]

′.The unconditional variance of ξt is

P ≡ var (ξt) =[

σ2z σzψ

σzψ σ2ψ

]. (27)

Given this informational structure, the monetary authority infersthe sources of commodity price fluctuations by solving a signal-extraction problem using a Kalman filter, i.e.,

Emat [zt ψt]

′ = Mqt, (28)

where M = PH [H ′tPH]−1 is a weighted average of the variances

and covariances of zt and ψt; M is calculated as

M =x

x2 − 2ρx + 1

[ρ − x1x − ρ

], (29)

where ρ = corr (zt, ψt) and x = σψ/σz.19

Three cases of equation (28) shed light on the tradeoffs facingthe monetary authority. In the first case (type A), when x → 0, thevolatility of the commodity supply shock is high relative to that of

18An online appendix, available at http://www.ijcb.org, also describes theBayesian learning approach to the problem.

19See chapter 13 of Hamilton (1994) for the derivation.

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Vol. 16 No. 2 Monetary Policy, Commodity Prices, Misdiagnosis 67

the commodity market markup. In this case, the monetary authorityattributes nearly all of the fluctuations to demand shocks. That is,

if x → 0, Emat [zt ψt]

′ → [0 qt]′.

In the second case (type B), the fluctuations are attributed to supplyshocks. That is,

if x → ∞, Emat [zt ψt]

′ → [−qt 0]′ .

In the last case (type C), the monetary authority attributes thecommodity price fluctuations partially to each component of thecommodity price, taking into account the relative volatility andcorrelation in equation (29).

Armed with these expectations, we can rewrite the monetaryauthority’s policy rule as

rt = Emat|t−1 [re

t + ϕcoreπY,t + ϕyyet ] + ϕcomΔqt

= Et|t−1 [ret + ϕcoreπY,t + ϕyye

t ] + ϕcomΔqt + et,(30)

where

et =[Ema

t|t−1 (ret ) − Et|t−1 (re

t )]

+ ϕcore

[Ema

t|t−1 (πY,t) − Et|t−1 (πY,t)]

+ ϕy

[Ema

t|t−1 (yet ) − Et|t−1 (ye

t )].

The et corresponds to a misdiagnosis error, which is endogenous,and Ema

t|t−1 denotes the expectations under the incorrect diagnosison the source of the shock. Note that when the monetary authorityimputes the change in commodity prices to the wrong type of shock,its estimates of endogenous variables will be incorrect, leading topersistent errors in interest rate setting.20

To investigate the implications of the endogenous error, wereplace equation (25) in our baseline model with equation (30). Theresulting impulse responses highlight the findings.

Figure 5 shows the impulse responses to a commodity supplyshock in the misdiagnosis case A. Even though the commodity price

20If the monetary authority correctly identifies the source of the shock, theerror is zero.

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Figure 5. Responses to a Negative Supply Shock underMisdiagnosis Type A

Notes: Impulse responses to a negative commodity supply shock when themonetary authority attributes all the fluctuation in the commodity price to anaggregate-demand shock (misdiagnosis type A). Other details are in figure 1.

is driven by a supply shock, the monetary authority misdiagnoses itas a traditional demand-driven commodity shock. If the monetaryauthority fails to recognize that an increase in the commodity priceis driven by (external) supply conditions, the consequence is overlytight monetary policy accompanied by a sizable drop in both outputand inflation. The commodity price responds in a more muted waythan in the baseline case because of tighter monetary policy. Coreand headline inflation both fall because of the slowing economy.

Figure 6 displays the impulse responses to a conventional aggre-gate demand shock in the case of misdiagnosis type B, i.e., whenthe rise in the commodity’s price is mistakenly attributed to a nega-tive commodity supply shock. In this case, the easier monetary pol-icy associated with the looking-through-the-supply-shock strategy

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Figure 6. Responses to a Positive Aggregate-DemandShock under Misdiagnosis Type B

Notes: Impulse responses to an aggregate-demand shock when the monetaryauthority attributes all the fluctuation in the commodity price to a supply shock(misdiagnosis type B). Other details are in figure 1.

results in higher output and inflation. This type of policy misdiag-nosis induces a procyclical increase in the commodity price.

Figures 7 and 8 report the results for the case of misdiagnosistype C, in which the monetary authority implements the optimallyweighted response to the commodity price rise based on the histori-cal commodity demand and supply shocks. The standard deviationsof the shocks are calibrated to the empirical estimates by Filardo andLombardi (2014), which yields a ratio of about 1.5 (x in equation(29)). Consistent with misdiagnosis cases A and B, the monetaryauthority responds excessively to supply shocks and insufficiently todemand shocks: on net, monetary policy appears excessively pro-cyclical in the model.

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Figure 7. Responses to a Negative Supply Shock underMisdiagnosis Type C

Notes: Impulse responses to a negative commodity supply shock when the mon-etary authority attributes the fluctuation in the commodity price proportionallyto aggregate-demand shock and commodity supply shock, with weights given bythe ratio of their standard deviations (misdiagnosis type C). Other details are infigure 1.

The relative importance of these misdiagnosis risks can also beassessed by examining the welfare losses under different policy rules.The left-hand and right-hand panels of figure 9 plot, respectively,the welfare loss of the misdiagnosis types A and B cases (relative tothe case of correct identification). In the case where the monetaryauthority misinterprets a rise in the commodity price as supply dri-ven, the contraction in both output and core inflation is larger thanin the full-information case. And, in the case where commodity pricefluctuations are driven by global demand, the monetary authorityamplifies cyclical fluctuations and, as a result, destabilizes the econ-omy. Hence, it is no surprise that the expected welfare losses arealways greater than zero under misdiagnoses.

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Vol. 16 No. 2 Monetary Policy, Commodity Prices, Misdiagnosis 71

Figure 8. Responses to a Positive Aggregate-DemandShock under Misdiagnosis Type C

Notes: Impulse responses to an aggregate-demand shock when the monetaryauthority attributes the fluctuation in the commodity price proportionally toaggregate-demand shock and commodity supply shock, with weights given bythe ratio of their standard deviations (misdiagnosis type C), assuming the effi-cient benchmark policy rule and a 0.5 autoregression coefficient for the shockprocesses.

With respect to the performance of core and headline policy rulesunder these cases of misdiagnosis, the results still lend support topolicy rules with core inflation rather than headline inflation. Inmisdiagnosis case A (supply shock treated as if it was demand),the core inflation rule implies a much lower expected loss than theheadline one. This is not surprising given the more muted policyresponse associated with the headline inflation rule. In the misper-ception B case, the difference in the expected loss between the tworules is small, with a slight edge to the headline rule. If misdiagno-sis risk cases A and B were both 50-50, the expected loss criteria

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Figure 9. Ratio of the Welfare Losses under Misdiagnosis

Notes: Welfare is computed using a second-order solution of the model, whereWt = U (Ct, Lt, et) + βEtWt and the welfare cost in terms of steady-state con-sumption is equivalent to {exp [(1 − β) (EWt − W ) − 1]} × 100.

would support the core inflation rule in this imperfect informationenvironment.

Overall, the signal-extraction results reinforce the earlier findingson the importance of correctly identifying the underlying nature ofcommodity price shocks. On the modeling side, the misdiagnosis riskand its inherent procyclicality lead to a breakdown of the divinecoincidence found in full-information models a la Blanchard andGalı (2007). On the policy side, the case for a core inflation rule isstrengthened.

4. Conclusions

In this paper we documented that (i) monetary authorities deliverbetter economic performance when they are able to accurately iden-tify the global nature of the shocks, i.e., global supply and demandshocks, driving commodity prices; and (ii) when it is difficult toidentify these shocks, monetary authorities can minimize some ofthe adverse feedbacks from misdiagnosis by targeting core inflation.Global shocks also imply that central banks may find themselvesresponding in a correlated way. If so, it is important to account forthis possibility. This reinforces arguments for greater prominence tobe given to global factors in domestic monetary policymaking.

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Vol. 16 No. 2 Monetary Policy, Commodity Prices, Misdiagnosis 73

One important aspect of monetary policy misdiagnosis risk thatdeserves further research includes the important issue of parame-ter uncertainty, especially with respect to the slope of the Phillipscurve. Indeed, recent empirical evidence suggests that the slope hasflattened, and at least has become more uncertain, across manyeconomies. In such a situation, the main call of this paper for moreattention to be given to the nature of the shocks hitting the econ-omy may take on even greater prominence. This may require goingbeyond the methods advocated in this paper, e.g., exploiting bigdata with cross-country coverage.

Appendix A. Model Calibration

Table A.1. Baseline Calibration

Structural Parameters Parameter Value

Share of Commodity in ConsumptionBasket

γ 0.10

Share of Commodity in ProductionFunction

α 0.10

Inverse Frisch Labor Supply Elasticity ν 0.50Price Elasticity of Substitution ε 7.66Quarterly Discount Factor β 0.99Price Adjustment Probability θ 0.75Size of Competitive Commodity

Production Relative to GDPX/Y 0.10

Notes: For the parameters associated with the commodity market, the choice is a bitmore arbitrary due to the less conventional view of the literature. The share of thecommodity in the consumption basket is set to 10 percent, which roughly matchesthe share of primary commodity inputs in the U.S. CPI. For the share of commoditiesin the production function, we also use 10 percent, as in Arseneau and Leduc (2013);given that we focus on commodities rather than oil alone, both values are larger thanthe 5 percent commonly used in oil-only models. Finally, the size of the competitivecommodity production sector relative to GDP is set at 10 percent.

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Appendix B. The Dominant CommodityExporter’s Problem

For the dominant commodity exporter, the optimization prob-lem can be written as a series of intratemporal decisions. Underthe assumption that the dominant commodity exporter does notfully internalize the actions of the other exporters (i.e., taking asgiven the macroeconomic variables (Ct, MCt, Yt, Δt, and Ωt) of thecommodity-importing country), the problem can be written as

maxMt

ln(Q1/(1−γ)

Mt − Mt/Zt

)

s.t. Qt = h(Mt).

The first-order condition of this problem is

Qt =[Z−1

t

11 − ζ−1ηt

,

where ηt ≡ −∂ lnQt/∂ lnMt = −h′ (Mt) Mt)/Qt is the elasticity ofcommodity demand (in absolute value) and ζ = 1 − γ.

The demand for the commodity by the dominant commodityproducer takes the form

Mt =1Qt

Dt − QtEt,

where Dt ≡ (γCt + αMCtYtΔt) and Et ≡ ΩtZt.The inverse demand function is

Qt =12

√M2 + 4DtEt − M

Et.

Given this, the elasticity of demand for the commodity is

ηt = −f ′ (Mt) Mt

Qt=

Mt√M2

t + 4DtEt

=Mt√

M2t + 4(Mt + Xt)Xt

=Mt

Mt + 2Xt.

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Vol. 16 No. 2 Monetary Policy, Commodity Prices, Misdiagnosis 75

Finally, the commodity price markup is

Ψt =1

1 − ηt= 1 +

Mt

2Xt.

Appendix C. Output, Inflation, and Interest RateBenchmarks

C.1 Efficient Output Gap Benchmark

The benchmark output gap can be derived by substituting theequations for labor demand, labor supply, aggregate demand, andcommodity demand for production into the aggregate productionfunction. The level of output in terms of marginal costs, the disper-sion of prices, productivity, and the real commodity price is

Yt =(

At

Δt

)1/(1−α)[

(1 − α) MCt Δt

(Qt)−γ/(1−γ) − αMCt Δt

]1/(1+v)

×(

αMCt Δt

Qt

)α/(1−α)

.

The log-linear approximation of the level of output, in deviationsfrom the steady state, is21

yt =1

1 − αat +

α

1 − α(mct − qt) +

11 + v

Υ(

mct +γ

1 − γqt

),

where Υ ≡[1 − α

μQγ/(1−γ)]−1

≥ 1.The level of efficient output, ye

t , is defined with respect to theefficient allocation, i.e., flexible prices and no monopolistic distor-tions in the commodity market or in the final goods market (whichimplies that Qe

t = Z−1t and μe = 1):

yet =

11 − α

at +(

α

1 − α− 1

1 + vγ

1 − γΥe

)zt,

21 Note that a linear approximation of the price dispersion, Δt, does not appearin this equation because price dispersion is assumed to have only second-ordereffects on the dynamics, as shown in Benigno and Woodford (2005).

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where Υe =[1 − αZ−γ/(1−γ)

]−1. The relationship between Ye and

Υ depends on the extent of monopolistic distortions. Ye and Υ areequal only if both markets are perfectly competitive or if the com-modity is not used for production (that is, α = 0).22 A key differenceis that commodity shocks affecting the markup do not have an effecton the level of efficient output: such output is affected only by fluc-tuations associated with supply shocks in the commodity market.As a consequence, a demand-driven increase in the commodity pricewould leave the benchmark efficient output gap unchanged. How-ever, a negative commodity supply shock would decrease the efficientoutput level.

The efficient output gap, yet , which is defined as the difference

between actual output and the efficient level of output, is of theform

yet = yt − ye

t .

C.2 Inflation Benchmarks

Expressed in log-linear terms, the equations for headline inflationand core inflation, respectively, are of the form

πt = πY,t +γ

1 − γΔqt

πY,t = κyyet + EtπY,t+1 + ut,

where ut is an endogenous cost-push shock, which is a function ofboth ψt and zt.

In this model, the divine coincidence featured in models withexogenous commodity prices is broken. It is no longer possible tosimultaneously stabilize core inflation and the welfare-relevant out-put gap. The tradeoff arises from the effect of commodity prices onthe level of efficient output. An increase in commodity price markupsgenerates a positive cost-push shock, which puts upward pressure oncore inflation but lowers the efficient output gap.

22More precisely, Υe > (<) Υ, if Ψγ/(1−γ) < (>) μ, where Ψ and μ are themarkups in steady state of the commodity and final goods markets, respectively.

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Vol. 16 No. 2 Monetary Policy, Commodity Prices, Misdiagnosis 77

C.3 Interest Rate Benchmarks

The interest rate benchmarks are derived by substituting the equa-tions for the aggregate resources constraint and the definition of theprice level into the IS equation:

1Rt

= βEt

[1

ΠY,t+1

(1 − αMCtQ

γ/(1−γ)t Δt

1 − αMCt+1Qγ/(1−γ)t+1 Δt+1

)

×(

Yt

Yt+1

)exp (gt+1 − gt)

].

The efficient interest rate is defined in the case where commodityand final goods markets are perfectly competitive and in this modelcan be written as

ret = (gt − Etgt+1) −

(ye

t − Etyet+1

)− γ

1 − γ(Υe − 1) (zt − Etzt+1) .

Note that the efficient interest rate does not respond to changes inthe markup.

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