Why Use Randomized Evaluation?

Presentation by Shawn Cole, Harvard Business School and J-PAL

Presenter: Felipe Barrera-Osorio, World Bank

APEIE WorkshopGhana, May 10-14

Fundamental Question

• What is the effect of a program or intervention?– Does microfinance reduce poverty?– Does streamlining business registration encourage

entrepreneurship?– Does auditing reduce tax evasion?

Explaining to grandparents• Nicholas Kristoff, New York Times Columnist (11/20/2009)

– “One of the challenges with the empirical approach is that aid organizations typically claim that every project succeeds. Failures are buried so as not to discourage donors, and evaluations are often done by the organizations themselves — ensuring that every intervention is above average. Yet recently there has been a revolution in evaluation, led by economists at Poverty Action Lab at MIT.

– The idea is to introduce new aid initiatives randomly in some areas and not in others [or to some people and not to others], and to measure how much change occurred and at what cost. This approach is expensive but gives a much clearer sense of which interventions are most cost-effective.”

Objective

• To Identify the causal effect of an intervention– Identify the impact of the program

• Need to find out what would have happened without the program– Cannot observe the same person with

and without the program at the same point of time

Correlation is not causation

Higher profits

Credit Use

OR ?

2)

?

1) Higher profits

Business Skills

Credit

Question: Does providing credit increase firm profits?

Suppose we observe that firms with more credit also earn higher profits.

2007 20090

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Treatment GroupTreatment Group

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(+6) increase in gross operating margin

Illustration: Credit Program (Before-After)

A credit program was offered in 2008.

Why did operating margin increase?

Motivation• Hard to distinguish causation from correlation by

analyzing existing (retrospective) data– However complex, statistics can only see that X moves

with Y– Hard to correct for unobserved characteristics, like

motivation/ability– May be very important- also affect outcomes of interest

• Selection bias a major issue for impact evaluation– Projects started at specific times and places for particular

reasons– Participants may be selected or self-select into programs– People who have access to credit are likely to be very

different from the average entrepreneur, looking at their profits will give you a misleading impression of the benefits of credit

Before After0

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4

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14Control GroupTreatment Group

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(+4) Impact of the program

(+2) Impact of other (external) factors

Illustration: Credit Program(Valid Counterfactual)

* Macroeconomic environment affects control group* Program impact easily identified

Experimental Design

• All those in the study have the same chance of being in the treatment or comparison group

• By design, treatment and comparison have the same characteristics (observed and unobserved), on average– Only difference is treatment

• Yields unbiased impact estimates

Medical Trials Analogy

• Medical trials:– Take 1,000 subjects– Assign 50% to treatment group, 50% to control– On average

• Age in treatment and control group the same• Pre-existing health in both groups the same• Expected evolution of health in both groups the same

– Track outcomes for treatment and control groups– “Gold standard” of scientific research

• Development projects– Many projects amenable to similar design

Options for Randomization

• Lottery (0nly some receive)– Lottery to receive new loans

• Random phase-in (everyone gets it eventually)– Some groups or individuals get credit each year

• Variation in treatment – Some get matching grant, others get credit, others

get business development services etc

• Encouragement design– Some farmers get home visit to explain loan

product, others do not

Lottery among the qualified

Must receive the program

Not suitable for the program

Randomize who gets the program

Opportunities

• Budget constraint prevents full coverage– Random assignment (lottery) is fair and

transparent• Limited implementation capacity

– Phase-in gives all the same chance to go first

• No evidence on which alternative is best– Random assignment to alternatives

with equal ex ante chance of success

Opportunities for Randomization

• Take up of existing program is not complete– Provide information or incentive for some to

sign up- Randomize encouragement

• Pilot a new program– Good opportunity to test design before

scaling up

• Operational changes to ongoing programs– Good opportunity to test changes before

scaling them up

Different levels you can randomize at

– Individual/owner/firm– Business Association– Village level

– Women’s association

– Regulatory jurisdiction/ administrative district

– School level

Group or individual randomization?

• If a program impacts a whole group-- usually randomize whole community to treatment or comparison

• Easier to get big enough sample if randomize individuals

Individual randomization Group randomization

Unit of Randomization• Randomizing at higher level sometimes

necessary:– Political constraints on differential treatment

within community– Practical constraints—confusing to implement

different versions– Spillover effects may require higher level

randomization

• Randomizing at group level requires many groups because of within community correlation– Micro-credit program to treat 100,000 people.

Choose Senegal and Gambia, and randomly offer program in one country.

– What do we learn?– Similar problem if choose only 4 or only 10

districts

Elements of an experimental design

Random assignmentTreatment Group Control Group• Participants Non-participants

Evaluation sample

Potential participantsTailors Furniture manufacturers

Target population

SMEs

External and Internal Validity (1)• External validity

– The evaluation sample is representative of the total population

– The results in the sample represent the results in the population We can apply the lessons to the whole population

• Internal validity– The intervention and comparison groups

are truly comparable – estimated effect of the

intervention/program on the evaluated population reflects the real impact on that population

External and Internal Validity (2)

• An evaluation can have internal validity without external validity

– Example: A randomized evaluation of encouraging informal firms to register in urban areas may not tell us much about impact of a similar program in rural areas

• An evaluation without internal validity, can’t have external validity

– If you don’t know whether a program works in one place, then you have learnt nothing about whether it works elsewhere.

Internal & external validity

Random Sample- Randomization

Randomization

National Population National Population

Representative Sample of National

Population

Representative Sample of National

Population

Internal validity

Stratification

Randomization

PopulationPopulation

Population stratumPopulation stratumSamples of Population

StratumSamples of Population

Stratum

Example: Evaluating a program that targets women

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Representative but biased: useless

National Population National Population

Non-random assignment

USELESS!

Non-random assignment

USELESS!

Randomization

Example: Randomly select 1 in 100 firms in Senegal.Among this sample, compare those with bank loans to those without.

Efficacy & Effectiveness

• Efficacy– Proof of concept– Smaller scale– Pilot in ideal conditions

• Effectiveness– At scale– Prevailing implementation arrangements --

“real life”

• Higher or lower impact?• Higher or lower costs?

Advantages of “experiments”

• Clear and precise causal impact• Relative to other methods

– Provide correct estimates– Much easier to analyze- Difference in

averages– Easier to explain– More convincing to policymakers– Methodologically uncontroversial

Randomly assigning machines within a plant to receive regular maintenance

Machines do NOT• Raise ethical or practical concerns about

randomization• Fail to comply with Treatment• Find a better Treatment• Move away—so lost to measurement• Refuse to answer questionnaires

Human beings can be a little more challenging!

What if there are constraints on randomization?

• Some interventions can’t be assigned randomly

• Partial take up or demand-driven interventions: Randomly promote the program to some– Participants make their own choices about

adoption

• Perhaps there is contamination- for instance, if some in the control group take-up treatment

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Randomly Assigned Marketing(Encouragement Design)

• Those who get receive marketing treatment are more likely to enroll

• But who got marketing was determined randomly, so not correlated with other observables/non-observables– Compare average outcomes of two groups: promoted/not

promoted– Effect of offering the encouragement (Intent-To-Treat)– Effect of the intervention on the complier population (Local

Average Treatment Effect)• LATE= ITT/proportion of those who took it up

RandomizationAssigned to treatment

Assigned to control

Difference Impact: Average treatment effect on the treated

Non-treated

Treated

Proportion treated

100% 0% 100%Impact of assignment

100%

Mean outcome

103 80 23Intent-to-treat estimate

23/100%=23Average treatment on the treated

Random encouragementRandomlyEncouraged

Not encouraged

Difference Impact: Average treatment effect on compliers

Non-treated(did not take up program)

Treated(did take up program)

Proportion treated

70% 30% 40%Impact of encouragement

100%

Outcome 100 92 8Intent-to-treat estimate

8/40%=20Local average treatment effect

Common pitfalls to avoid

• Calculating sample size incorrectly– Randomizing one district to treatment and

one district to control and calculating sample size on number of people you interview

• Collecting data in treatment and control differently

• Counting those assigned to treatment who do not take up program as control—don’t undo your randomization!!

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When is it really not possible?

• The treatment already assigned and announced

and no possibility for expansion of treatment

• The program is over (retrospective)• Universal take up already• Program is national and non excludable

– Freedom of the press, exchange rate policy(sometimes some components can be

randomized)

• Sample size is too small to make it worth it

Further Resources

• Google “Impact Evaluation World Bank”• DIME group: www.worldbank.org/dime • JPAL, IPA• Email presenter: fbarrera@worldbank.org

Thank You