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ContemporaryIssuesinCrimeandJustice Number93
January2006
DonWeatherburn,JiuzhaoHua&SteveMoffatt
Thisbulletinpresentstheresultsofastudyintotheincapacitationeffectofprisononburglary.Theresultsindicate that current levelsof imprisonment inNewSouthWales (NSW)preventapproximately45,000burglariesperannum.Ratesofburglarycouldbereducedifsentencesforburglarywerelonger,ahigherpercentageofburglarsweresenttoprisonorclear-upratesforburglarywerehigher.Theeffectivenessofthesemeasureswouldbereduced,however,iftheyresultedinfewerguiltypleas,higherre-offendingratesortheentryofmoreoffendersintothestolengoodsmarket.Increaseduseofimprisonmentmaynotbeaverycost-effectivewayofreducingburglary.Togeta10percentreductioninthecurrentburglaryrateviaimprisonmentthenumberofburglarssentencedtoprisoninNSWwouldhavetobeincreasedbyatleast34percent.Thiswouldcostanadditional$26millionperyear.Thebulletinconcludesbycallingformoreresearchintothecost-effectivenessofprisonanditsalternativesincontrollingcrime.
IntroductIon
Inthedecadebetween1995and2004,
theAustralianprisonpopulationincreased
bymorethan39percent(Australian
BureauofStatistics2004a).Onany
givendaytherearenowmorethan
25,000peopleheldinAustralianprisons
(AustralianBureauofStatistics2005).
Thecostofkeepingthismanypeoplein
prisonissubstantial.Lastfinancialyear
therecurrentcostofimprisonmentin
Australiaexceeded$1.6billion,orabout
$92perheadofpopulationperannum
(ProductivityCommission2005).Prison
maynotbethemostcommonsanction
foroffendingbutitisundoubtedlythe
mostexpensive.Itisalsoonethatcourts
inNewSouthWales(andperhapsother
Statesaswell)haveincreasinglybeen
turningtoindealingwithpersistentand/or
seriousoffending(Fitzgerald2001).
Surprisinglylittleresearchhasbeen
conductedinAustraliaintotheeffectof
prisononcrime.Thisisveryunfortunate.
Imprisonmentmaybeaveryblunt
instrumentofcrimecontrolbutitisan
importantinstrument,nonetheless.
Thefactthatsomuchpublicmoneyis
spentonimprisonmentonlyservesto
underscoretheneedforacarefuland
thoroughassessmentofitseffects.The
purposeofthisbulletinistopresent
theresultsofastudyintotheeffectof
imprisonmentonburglary.Theremainder
ofthebulletinisdividedintofoursections.
Inthenextsectionweintroducesomekey
conceptsanddiscusstheresultsofearlier
researchintotheeffectivenessofprison.
Inthesectionthatfollowswedescribe
themethodsanddatasourcesused
forouranalysis.Inthethirdsectionwe
presentourresults.Inthelastsectionwe
summariseanddiscussourfindings.
Past research
studIes of the ImPrIsonment/crIme correlatIon
Intheory,prisoncouldinfluencecrime
eitherthroughdeterrenceand/or
incapacitation.Deterrencereferstothe
crimepreventioneffectthatresultsfrom
fearofbeingsanctionedforoffending.
Incapacitationreferstothecrime
preventioneffectthatresultsfromkeeping
offenderslockedupandthereforeunable
tooffend.Deterrencecantaketheformof
specificdeterrence:theeffectapenalty
hasonoffendingbythepersononwhom
itisimposed,orgeneraldeterrence:the
generaleffectthatpenaltieshaveonthe
generalwillingnessofpeopletooffend.
Criminologistshavetraditionallybeen
somewhatscepticalaboutthecapacity
ofprisonstoinfluencecrimethrough
How much crime does prison stop?The incapacitation effect of prison on burglary
CRIME AND JUSTICEBulletin NSW Bureau of Crime
Statistics and Research
BUREAUOFCRIMESTATISTICSANDRESEARCH
2
deterrenceorincapacitation.Accordingtoonecommonlycitedargument,forexample:
“…forevery1,000crimescommittedinAustralia, 400 are reported topolice,320arerecordedbypoliceas crimes, about 64 result in thedetectionofanoffender,43 resultin convictions and 1 person isgaoled.” (Mukherjee, Walker,Psaila, Scandia & Dagger 1987)
Doublingoftheprisonpopulation,onthisaccount,wouldaffectonlyaboutonetenthofonepercentofcrimescommitted.Itdoesnotfollowfromthefactthat64peoplearearrestedforevery1,000offences,however,that936offendersgetoffwithoutbeingapprehendedandpunished.Manyoffenderscommitlargenumbersofoffencesandhavelongcriminalcareers.Thesepeopleaccountforadisproportionateamountofalloffending(Blumstein,Cohen,Roth&Visher1986).Imprisoningevenasmallproportionofthemmightexertadisproportionateeffectoncrime.Intheabsenceofresearchevidencedemonstratingitsineffectiveness,then,itwouldbewrongtoassumethatprisonexertsnoeffectoncrime.
Thereisresearchthatgivesuscausetodoubtthedeterrenteffectivenessofimprisonment.Anumberofstudieshavefoundthat,unlesstheperceivedriskofapprehensionisfairlyhigh,thethreatoftougherpenaltiesdoesnotexertmuchdeterrenteffectonthestatedwillingnessofpeopletobecomeinvolvedinaparticularoffence(Howe&Loftus1996).Thisevidenceisconsistentwithstudiesofthespecificdeterrenteffectoftougherpenalties,manyofwhichfindeithernoeffectorinconsistenteffects(Spohn&Halloren2002;Smith&Akers1993;Gottfredson1999;Briscoe2004;Dejong1997).Mostattemptstoassesstheeffectivenessofprison,however,makenoassumptionsaboutwhetherprisonexertsitseffectsviadeterrenceorincapacitation.Theysimplyexaminethecorrelationbetweencrimeandsomemeasureofpenalseverity(e.g.imprisonmentrates)whilecontrolling
forotherfactorsthatmightinfluence
crime.Ifimprisonmentdoesreduce
criminalbehaviour,ratesofcrimeand
imprisonmentshouldbenegatively
correlatedafterotherrelevantfactorshave
beentakenintoaccount.
Earlystudiesofcrimeandimprisonment
ratesobtainedinconsistentfindingsonthis
issue.Manyofthesestudies,however,
madenoattempttodealwiththeproblem
ofsimultaneity:thereciprocalrelationship
betweencrimeandcriminaljusticeactivity
(seeBlumstein,Cohen&Nagin1978).
Whencrimeratesincreaseweexpect
policetoarrestmoreoffenders(Listokin
2003)andcourts,asaconsequence,to
putmoreoffendersinprison.Thispattern
ofrisingcrimeandrisingimprisonment
ratesmayhidewhateverpreventative
effectprisonhasoncrime.Severalstudies
publishedinthelastdecadehavefound
waysofadjustingforsimultaneityand
theyprovideconsistentevidencethat
incarceratingoffendersdoesexerta
significantsuppressioneffectoncrime.
AccordingtoSpelman(2000),thebest
estimatesoftheeffectonseriouscrimeof
a10percentincreaseinimprisonmentin
theUnitedStatesrangebetween1.6and
3.1percent(Spelman2000).
Becausecrimeandimprisonmentrates
varyfromonecountrytoanotherwe
cannotsafelyassumethattheresultsof
overseasstudiesontheeffectiveness
ofprisonsautomaticallyapplyhere.
Onlyafewstudies,however,have
everbeenconductedinAustraliainto
theeffectofprisononcrime.Withers
(1984)conductedthefirst,usingdataon
recordedcrimeratesintheAustralian
StatesandTerritoriesovertheperiod1964
to1976.Heexaminedtheeffectofrates
ofapprehension(asmeasuredbytheratio
ofcourtcommittalstorecordedcrimes)
andimprisonment(asmeasuredasthe
ratioofprisonerstocourtcommittals),on
ratesofvariouskindsofcrime,controlling
forarangeofotherfactors(e.g.income,
unemployment,education)thatmightbe
expectedtoinfluencecrime.Hisanalysis
indicatedthathigherratesofimprisonment
areassociatedwithlowerratesofproperty
crimebutnotwithlowerratesofwhat
hecalled‘crimesofpassion’,suchas
homicideandsexualassault.Withers’analysisindicatedthata10percentincreaseinimprisonmentwouldreducepropertycrimebybetween5.1and6.2percent.
Inalaterstudy,BodmanandMaultby(1997)updatedandextendedWithers’(1984)analysis,makingthreesignificantimprovements.First,theymeasuredtheeffectofimprisonmentusingexpectedsentencelengthratherthanthenumberofoffendersimprisoned,arguingthatthisprovidedamoresensitivemeasureoftheeffectoftougherprisonpenaltiesoncrime.Second,theymadeadjustmentsintheiranalysisforthereciprocalrelationshipbetweencriminaljusticeactivityandcrime.Third,theyusedamoreextensivedatasetthanWithershadbeenabletouse.Theyfoundevidencethatlongerprisonsentenceswereassociatedwithlowerratesofrobbery,motorvehicletheftandfraud.However,unlikeWithers,theydidnotfindanyeffectofimprisonmentonburglary.Theiranalysisindicatedthata10percentincreaseinprisonsentencelengthswouldreducerobbery,motorvehicletheftandfraud,bybetween3.8and5.2percent.
SincetheBodmanandMaultby(1997)study,twootherAustralianstudieshavebeenconductedwhich,whilenotdirectlyconcernedwiththeeffectofprisononcrime,havenonethelessyieldedevidenceofitseffects.
ChilversandWeatherburn(2003)examinedtheeffectofheroindependenceonlong-termrobberytrends,controllingforchangesinunemployment,heroinuse,robberyclear-upratesandratesofimprisonmentforrobbery.TheyfoundthattheriseinrobberyinNewSouthWalesbetween1966and2000wasstronglycorrelatedwithariseinheroinusebutitwasalsoindependentlyrelatedtoalong-termfallinratesofimprisonmentfortheoffence.1Inamorerecentstudy,Moffatt,WeatherburnandDonnelly(2005)examinedtrendsinburglaryandrobberyinNewSouthWalesbetweenJanuary1998andOctober2003.Theyfoundthatlongeraggregateprisonsentenceswereassociatedwithlowerlevelsofburglary(butnotrobbery)
BUREAUOFCRIMESTATISTICSANDRESEARCH
3
aftercontrollingfortreatmententry,druguse,unemployment,consumerspendingandarrestrates.Theirresultsindicatedthata10percentincreaseinaggregateprisontimewouldreduceburglaryby6.3percent.
Noneofthesestudiesisentirelyimmunetocriticism.NeitherChilversandWeatherburn(2003)norMoffattetal.(2005)includedcontrolsforsimultaneity.Withers(1984)andBodmanandMaultby(1997),ontheotherhand,didnotcontrolfortrendsinheroindependence,afactorthatChilversandWeatherburn(2003)hadfoundtobestronglylinkedtotrendsinpropertycrime.Omittingimportantvariablesisnotaproblemwhentheomittedvariablesarenotstronglycorrelatedwiththosewhoseeffectsarebeingmeasured.When,however,theomittedvariablesarecorrelatedwithimprisonmentestimatesoftheeffectofimprisonmentoncrimecangivebiasedandmisleadingresults.Imprisonment,ineffect,endsupactingasaproxyforotherfactorsthatinfluencecrimebutwhichhavenotbeenincludedintheanalysis(seeSpelman2000,p.440).
IncaPacItatIon studIes
ThedifficultiesinvolvedinadequatelycontrollingforextraneousfactorshavetemptedsomeresearcherstotakewhatSpelman(2000)hascalleda‘bottomup’approachtoestimatingtheeffectofprisononcrime.Insteadoflookingatthecorrelationbetweentherateofoffendingandtherateofimprisonment,theyestimateitseffectusingamathematicalmodeldevelopedbyAvi-ItzhakandShinnar(1973)andShinnarandShinnar(1975).Thismodelassumesthereisafinitepopulationofoffenderswho,whentheyarefreeinthecommunity,commitcrimeatacertainrateandremaininvolvedincrimeoveracertainperiodoftime(knownastheircriminalcareer).Accordingtothemodel,thelargerthefractionofanoffender’scriminalcareerspentinprison,thelesscrimetheyareabletocommit.
Theamountofcrimepreventedbyprisoninthemodeldependsonfivethings:(1)therateatwhichoffenderscommitcrimewhenfree,(2)thelikelihoodof
anoffenderbeingcaughtandconvicted,
(3)thelikelihood,ifconvicted,thatan
offenderwillreceiveaprisonsentence,
(4)theaveragetimespentinprisonand
(5)thelikelihoodofanoffenderresuming
hisorherinvolvementincrimeonceheor
sheisreleasedfromprison.Equation(1)
below,describesthepreciserelationship
Avi-ItzhakandShinnarderivedconcerning
therelationshipbetweentheamountof
crimepreventedandthesefivefactors:
I = λqJS {TR / (TR + S)}
1 + λqJS {TR / (TR + S)}
where:
I = thefractionofcrimesavoidedasaresultofincapacitation
λ = therateatwhichoffenderscommitcrimes
q = theprobabilityofbeingapprehendedandconvictedforacrime
J = theprobabilityofbeingsentencedtoprisonifconvicted
S = theaveragetimespentincustody
TR = theaveragetimeoffenderswillremaininvolvedincrime
TheparameterImeasurestheamount
ofcrimepreventedbythecurrentlevel
ofimprisonment.Itcanbethoughtof
asthepercentageincreaseincrime
thatwouldresultifalloffenders(or
alloffendersofacertaintype)were
released.Note,however,thatImustbe
adjusteddownwardstoaccountforthe
factthat,whenco-offenderscommita
crime,imprisoningbothwillonlysaveone
offence(Blumsteinetal.1986,p.60).2
TheShinnarandShinnarmodelcan
beusedtoderiveanequationforthe
percentagechangeintheannualcustodial
populationrequiredtoachieveaoneper
centchangeinthelevelofcrime.This
change,knownastheelasticity(E)of
crimeinrelationtoprison,isgivenby:
E = 1 + λqJS2TR / (TR + S)2 - λqJSTR
2/ (TR + S)2
JustasthevariableIhastobeadjustedtoaccountforco-offending,Ealsohastobeadjustedforthesameeffect(seemethodsectionbelow).
Theadvantageoftheincapacitationapproachisthatitsidestepstheproblemofhavingtoworkoutwhattocontrolforwhenlookingattheeffectofprisononcrime.Likeallmodels,however,themodelofincapacitationdevelopedbyAvi-ItzhakandShinnar(1973)andShinnarandShinnar(1975)restsonanumberofassumptions.Therearefourinparticularthatdeservemention:
Inanyapplicationofthemodel,accurateestimatesofthemodelhavebeenobtained.
Alloffendersruntheriskofbeingarrestedandincarcerated.
Themoreoffendersweimprison,thefewerthereareinthegeneralpopulation
Theexperienceofimprisonmentdoesnotchangetheexpectedlengthofacriminalcareer(TR)ortherateatwhichindividualsoffend(λ).
Wewillreturntotheseassumptionswhenwediscussourresults.
Moststudiesofincapacitationsuggestthatprisonexertsasignificantsuppressioneffectoncrime;however,theestimatedeffectsappeartovarymarkedlyfromstudytostudy.Blumsteinetal.,forexample,citeevidencethatthelevelofimprisonmentprevailingintheUnitedStates(US)duringthe1970swouldhavehadanincapacitationbenefitof20percent(Blumsteinetal.1986,p.123).AstudyofincapacitationintheUnitedKingdombyTarling(1993),however,puttheincapacitationeffectofprisoninthatcountryinthemid-1980satbetween7.3and9.0percent.AlthoughtheestimatesreportedbyBlumsteinandTarlingdiffersignificantly,mostincapacitationstudiesconcludethatlargeincreasesintheprisonpopulationonlyproducefairlymodestreductionsincrime.ResearchintheUnitedStates,forexample,suggeststhatinmostUSstatestoobtaina10percentreductionincrime,theprisonpopulationwouldhavetobemorethan
doubled(Chan1995,p.6).
1.
2.
3.
4.
(1)
(2)
BUREAUOFCRIMESTATISTICSANDRESEARCH
4
the Present study
Thefactthatincapacitationestimates
varysosignificantlybetweenBritain
andtheUnitedStatessuggeststhatto
obtainreliableinformationaboutthe
incapacitationeffectofprisoninAustralia
weneedtoconductourownresearch.
Thisisdifficulttodobecauseverylittle
researchhasbeenconductedinthis
countryonhowfrequentlydifferenttypes
ofoffenderscommitcrimeorhowlong
differentgroupsofoffendersspend
involvedincrime.
Fortunately,dataonoffendingfrequency
andcriminalcareerlengthcanbe
obtainedforatleastoneoffence.
Salmelainen(1995)conductedastudy
of247juveniletheftoffendersheldin
NSWdetentioncentres.Sheaskedher
respondentswhethertheyhadever
committedaburglaryand,iftheyhad,
howmanytheyhadcommittedinthesix
monthspriortothearrestthatresulted
intheirincarceration.Theiranswerscan
beusedtoestimateoffendingfrequency.
Wecanobtainanestimateofresidual
criminalcareerlength,ontheotherhand,
fromdatacollectedaspartofastudy
ofre-offendingamongNSWparolees
conductedbyJones,Hua,Donnelly,
McHutchisonandHeggie(2005).They
examinedthere-offendingratesofa
groupofmorethan2,000prisoners
releasedonparoleinthefinancialyear
2001-2002.Morethanfivehundredof
theseoffendershadbeenconvictedof
break,enterandsteal(i.e.burglary).
Theremainingdataneededforequation
(1)canbeextractedfromdatabases
maintainedbytheNSWBureauofCrime
StatisticsandResearch.
sources of data and methods
mean offendIng rate (λ)
Therearetwowaysofestimatingthe
parameterλ.Thesimplestandprobably
mostreliablemethodisthroughstudies
ofself-reportedoffendingfrequency.Only
twosuchstudieshavebeenconductedin
NewSouthWales,onebySalmelainen
(1995)andtheotherbyStevensonand
Forsythe(1998).Inthisstudywerelyon
Salmelainen’sdataforreasonsthatare
explainedindetailinthenotestothis
bulletin.3Sufficetosaythattheestimates
ofaverageoffendingfrequencybased
onStevensonandForsythe’sdataare
sohighandsoinconsistentwiththe
estimatesobtainedusingothermethods
(seebelow),theycannotberegardedas
credible.
Salmelainen’sdatashowthatthe
meannumberofburglariesperburglar
isapproximately68perannum.The
distributiononwhichtheaveragewas
based,however,wasextremelyskewed,
withoneoffenderclaimingtohave
committed700burglariesinthepreceding
sixmonths.Itishardtoseehowanyone
wouldhavetimetocommitthismany
burglaries(about4perday),letalone
remembereachonewellenoughtokeep
trackofthetotalnumbercommittedover
asixmonthperiod.Itislikelythatsuch
extremevaluesofoffendingfrequency
simplyreflectexaggerationonthepartof
therespondent.Ifwefollowtheprocedure
recommendedbyVisher(1986)for
dealingwithsuchcases,andtruncate
theoffendingfrequencydistributionat
the90thpercentile,Salmelainen’sdata
indicateameanoffendingfrequencyof
38.1burglariesperannum.Notethatthis
estimate,thoughhigh,isintherangecited
byBlumsteinetal.(1986,p.66)inthe
UnitedStates.
Itmayseemsomewhatarbitraryremoving
10percentofthesampleonwhich
ourestimateofoffendingfrequencyis
based.Asacheckonthereliabilityofour
estimateofoffendingfrequency,therefore,
weobtainasecondindependentestimate
usingtheequation:
μ=λp
whereμisthearrest(orcourt
appearance)rateofanindividualburglar,
λistherateatwhichtheburglarcommits
burglariesandpistheprobabilitythatany
particularoffenceresultsinanarrest(or
anappearanceincourt).Ifthisequation
isaccepted,thevalueofλisgivenbyμ/p.
Informationonμcanbeobtainedfrom
unpublishedcourtdataheldbytheNSW
BureauofCrimeStatisticsandResearch.
Thissourceshowsthattheaverage
numberofchargesofburglaryperperson
convictedofburglaryin2004was1.9.
Asnotedearlier,however,weneedto
adjustthisfiguredownwards,toaccount
forthefactthat,iftwooffenderscommit
oneburglary,imprisoningbothoffenders
willonlypreventoneburglary.Wedo
thisbydividingtheparameterμbythe
averagenumberofburglarsperburglary.
UnpublishedBureaucrimedatashow
thattheaveragenumberofoffendersper
burglaryincidentinNewSouthWalesin
2004was1.49.Thisgivesusanadjusted
valueofμ=1.28.
Wecanestimatepfrompolicedata
ontheannualpercentageofburglary
offencesclearedbypolice.Thissource
givesavalueof5.8percentforthe
180dayclear-uprateforhomeburglary
and6.2percentforthe180dayclear-
uprateforburglariesnotinvolving
dwellings(NSWBureauofCrime
StatisticsandResearch2005,p.37).
Wethereforeassumethattheoverall
clear-uprateissomewherearound6.0
percent.Thisclear-upratemustbe
adjusteddownwardstotakeaccount
ofthefactthatsomeburglariesare
notreportedtopolice.Tomakethis
adjustmentwemultiplytheclear-uprate
bythepercentageofburglariesreported
topolice.Crimevictimsurveydata
(AustralianBureauofStatistics2004b,
p.10)indicatethatin2004,67percentof
homeburglarieswerereportedtopolice.
Multiplying.06by.67,gives.04asour
adjustedclear-uprate.Dividing1.28by
.04givesanalternativeestimateofλof
32burglariesperyear.
Thesimilarityofthisestimatetothe
estimateobtainedinSalmelainen’sself-
reportstudyisveryreassuring.Allthe
same,theestimateobtainedviaequation
(3)islikelytobeanunderestimate
becauseitassumesthatnoonecharged
withburglaryin2004wasinprisonduring
thatyear.Wethereforetreat38.1asthe
morereliableestimateofλ.Asacheckon
thesensitivityIatoλ,weplotIaforarange
ofvaluesofλoneithersideof38.1.
(3)
BUREAUOFCRIMESTATISTICSANDRESEARCH
5
the ProbabIlIty of beIng aPPrehended and convIcted for burglary (q)
Theparameterqistheproductof(a)theprobabilitythatanoffencedetectedbypoliceleadstothearrestofanoffenderand(b)theprobabilitythattheoffenderisconvicted.Asnotedintheprevioussectionweusetheclear-uprateforburglaryinNSWasanestimateofthefirstoftheseprobabilities.UnpublishedcourtdataheldbytheNSWBureauofCrimeStatisticsshowthatin2004,76percentofpersonschargedwithaburglaryoffencewereconvictedofthatoffence.Accordinglyweassumeq=0.060*0.76=.045.
the ProbabIlIty that a convIcted burglar receIves a PrIson sentence (J)
UnpublishedcourtdataheldbytheNSWBureauofCrimeStatisticsandResearchshowthat,in2004,44percentofthepersonswhohadbeenchargedwithburglaryandeitherconvictedofburglaryorsomeotheroffence,4receivedaprisonsentence.AccordinglyweassumeJ=.44.
the average tIme (In years) sPent by burglars In custody (S)
UnpublishedcourtdataheldbytheNSWBureauofCrimeStatisticsandResearchshowthatin2004,theaverageminimumtermimposedbyNSWCourtsonpersonsconvictedofburglarywas1.02years.5Someoffendersarenotreleasedattheendoftheirminimumtermbutthefigureof1.02nonethelessaccordsverycloselywiththeestimatedtimespentincustodybyasampleof466burglarsreleasedtoparolesupervisioninthe2001-2002financialyear,andfollowedupbyJonesetal.(2005).Datadrawnfromthatstudyshowthatburglarsreleasedonparoleduringthisperiodhadspent,onaverage,1.01yearsincustodypriortotheirrelease.6AccordinglyweassumethatS=1.02years.
resIdual career length (TR)
TherearenodatathatcanbeusedtoobtainadirectestimateofTR.However,ifTRislargecomparedwithS(i.e.ifthe
expectedresidualcriminalcareerlengthislargecomparedwiththeexpectedsentencelength),TR / (TR + S) approaches1andequation1reducesto:
I = λqJS / (1 + λqJS)ShinnarandShinnaruseequation(4)toavoidtheproblemofestimatingTR.Ratherthanmakethisassumptionwetakeasomewhatdifferenttack.Itcanbeshownthat,ifthelengthofacriminalcareerisdistributedexponentially,withmeanresidualcareerlengthTR,andiftimeservedinprisonisalsoexponentiallydistributedwithmeanlengthS,thenTR / (TR + S)istheprobabilityPAthatanoffenderisstillactiveinacriminalcareerafterservingasentence(Tarling1993).InthiscasewecanestimateTR fromtheequationTR = PA S / (1 - PA).Thedistributionoftimetore-offendandtimeincustodyinNSWarebothreasonablywellapproximatedbyanexponentialdistribution(seeAppendix).TheparameterPAontheotherhand,canbeestimatedfromthestudybyJonesetal.referredtoearlier.Thatstudyfoundthat80.1percentofburglarsreleasedonparolehadre-appearedincourtwithinthefollowupperiodof27-39months.7PuttingthisvalueintotheequationforTRgivesavalueofTR=4.1years.
summary of Parameter values
Insummary,exceptwhereotherwiseindicatedweassume(a)thatimprisonedburglarscommitanaverageof38.1
burglariesperyearwhenfree,(b)thatthe
chanceofaburglarbeingarrestedand
convictedinthecourseofayearisabout
4.5percent,(c)that44percentofthose
convictedaregivenaprisonsentence,(d)
thattheaverageperiodspentincustody
bythoseimprisonedis1.02yearsand(e)
thattheaverageresidualcriminalcareer
foraburglarlasts4.1years.
results
Wearenowinapositiontopresentthe
resultsofouranalysis.Webeginby
presentingdataontheincapacitation
effectofprison.Thisisfollowedbyan
analysisoftheeffectsonburglaryof
(a)increasingtheaveragesentencefor
burglars,(b)increasingtheproportion
ofconvictedburglarssenttoprisonand
(c)increasingtheburglaryclear-uprate.
Wethenexaminethecostsassociated
withreducingburglaryviagreateruseof
imprisonment.
the IncaPacItatIon effect of PrIson
Figure1,below,showstheadjusted8
incapacitationeffect(Ia)ofprison
onburglaryinNSWasafunctionof
offendingfrequency(λ).Thepoint
atwhichthedashedhorizontalline
crossestheY-axisindicatesthelevel
ofincapacitationcorrespondingtoour
assumedvalueofλ(38.1).
(4)
Figure 1: Burglaries prevented by offending rate
0.0
5.0
10.0
15.0
20.0
25.0
30.0
31 32 33 34 35 36 37 38 39 40 41 42 43 44 45
Offending rate
Inca
paci
tatio
nef
fect
(per
cen
t)
BUREAUOFCRIMESTATISTICSANDRESEARCH
6
Figure1indicatesthatthecurrentimprisonmentrateforburglaryinNewSouthWaleskeepsthenumberofburglariesabout26percentlowerthanitwouldotherwisebe.Thisisequivalenttopreventingabout44,700domesticandcommercialburglaries.9Thisconclusionisnotoverlysensitivetothevalueofλweassume.Ifthetruevalueofλwere32offencesperyear,forexample,(i.e.thevalueofλobtainedusingequation(3)),theestimatedincapacitationeffectofprisononburglaryfallsto23percent.Ifthetruevalueofλwere44,ontheotherhand,theincapacitationeffectwouldonlyrisetoabout28percent.TheseestimatesarewellwithintherangereportedinBlumsteinetal.(1986)forburglaryoffenders.
Weturnnowtothequestionofwhether,andtowhatextent,furtherincreasesinimprisonmentwouldbringtheburglaryratedown.ToexplorethisissueweexaminetheeffectonI ofchangesin:S(sentencelength),J(theproportionofburglarssenttoprison)andthepercentageofburglariesclearedbypolice.
the effect of changIng sentence length
Figure2showstheestimatedincapacitationeffectofprison(Ia)onburglaryastheaveragesentencelength(S)forburglaryincreases.
Itcanbeseenthat,astheaveragesentencelengthincreasesfromoneyear(itscurrentlevel)towardtwoyears,theincapacitationeffectsteadilyincreasesfromabout26percent,toalittleover34percent.Inotherwords,iftheaveragetermofimprisonmentwereincreasedfromonetotwoyears,theburglaryratewouldfallbyabouteightpercentagepointsorabout10,188burglaries.
the effect of PuttIng more burglars In PrIson
Asecondwaytoincreasetheincapacitationeffectofprisonistoputmoreburglarsinprison.Figure3showstheeffectonIaofchangesinJ(the
probabilityofaprisonsentence).
Itcanbeseenthatastheprobabilityofaprisonsentenceincreasesfromabout44percenttoward88percent(i.e.doubleitscurrentvalue),theestimatednumberofburglariespreventedrisesfromabout26percenttoalittleover37percent,apreventiongainofapproximately11percentagepoints(orabout14,000fewerburglaries).Thetrueeffectofdoublingthelikelihoodofaprisonsentence,however,islikelytobemuchlowerthanthisestimatesuggests.Thisisbecauseoffendersinprisongenerallyhavehigheroffendingrates(whenfree)thanoffenderswhohavebeenarrestedbutnotdeemedtobepersistentenoughtodeserveaprisonsentence.Offenders
whohavenotbeenarrestedgenerallyhaveloweroffendingratesagain.Canela-Cacho,Blumstein,andCohen(1997),forexample,foundthatonlyonetofourpercentofrobbersinthecommunitycommitmorethan10robberiesperyear,butbetween24and48percentofimprisonedrobberscommitrobberiesatthisrate.Similarly,whileSalmelainen(1995)foundthatincarceratedjuveniletheftoffendersinNSWcommitburglariesattherateofaboutoneoffenceeverythreeweeks,Baker(1998)foundthatNSWsecondaryschoolstudentswhoadmittedinvolvementinburglarycommittedonlyaboutoneortwooffencesperyear.
Figure 3: Burglaries prevented by prison sentence probability
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95
Probability of prison sentence
Inca
paci
tatio
nef
fect
(per
cent
)
Figure 2: Burglaries prevented by sentence length
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1
Sentence length (years)
Inca
paci
tatio
nef
fect
(per
cent
)
BUREAUOFCRIMESTATISTICSANDRESEARCH
7
Theseconsiderationssuggestthat,asweputmoreburglarsinprison,theaveragefrequencyofoffendingamongthoseweincarceratewillfall.Thereisnowayofknowingpreciselyhowλwillfallbutwecanuseequation(3)abovetoobtainanestimateofλamongburglarswhoreachcourtbutarenotcurrentlysenttoprison.Therewere1,262individualsconvictedofburglaryinNSWin2004whowerenotsentencedtoatermofimprisonmentinthatyearandhadnotbeengivenasentenceofimprisonmentsinceatleast1994.10Thesepeopleaccumulatedanaverageof0.75chargesofburglaryperyearbetween2002and2004.Asexpected,thisismuchlowerthantheaveragenumberofchargesamongstconvictedburglarssenttoprison.Scalingthisfigureup(usingequation(3)above)givesavalueofλ=12.5burglariesperyear.Ifallimprisonedoffendersoffendedatthisrateandwedoubledtheimprisonmentrate,theincapacitationeffectofprisonwouldonlybeabout20percent.
the effect of changIng clear-uP rates
Theincapacitationeffectofprisononburglarywillalsoincreaseifeither(a)policeimprovetheirclearuprateforburglaryor(b)prosecutorsbecomemoresuccessfulatconvictingthosetheychargewithburglary.Therearenoeasywaysofincreasingtheconvictionrateforburglary,whichisinanyeventalreadyfairlyhigh.Theclear-uprateforburglaryisquitelowandmightbehigherifpolicehadtheresourcesrequiredtoinvestigateeachburglarymorethoroughly.Figure4showstheeffectofincreasingtheburglaryclearupratefromsixpercent(itscurrentlevel)to12percent.
Astheclear-upraterisesfromsixpercenttoabout12percent(i.e.doubleitscurrentvalue),theincapacitationeffectofprisonrisesfromabout26percenttoabout37percent.Note,however,thatthisisonlytrueifpolicecanincreasetheirclear-upratewithoutapprehendingoffenderswhoseburglaryrateissignificantlylowerthanourassumed38.1offencesperyear.Iftheclear-uprateroseto12percentbutintheprocessthe
averagefrequencyofoffendingamong
thosesenttoprisonfellto24offences
peryear(i.e.abouthalfwaybetweenthe
assumedoffendingrateofthosecurrently
senttoprisonandtheestimatedoffending
rateofburglarsbroughttocourtbutnot
currentlysenttoprison)theincapacitation
effectofprisonwouldonlyrisefrom26per
centto30percent.Thisisequivalenttoa
savingof5,094burglaries.
the cost of reducing burglary through increased imprisonment
Sofarwehaveonlyconsideredthe
benefitsofincapacitation.Everydrop
incrimeproducedbyanincreaseinincapacitation,however,comesatacostintermsofincreasedprisonernumbers.Thisraisesthequestionofhowmuchwewouldneedtopay(intermsofincreasedprisonnumbersandexpenditure)toachieveagivenpercentagereductionincrime.
Ifweassumethatλ=38.1,andthatallothervaluesoftheparametersareheldatthevaluesshownearlier,theadjustedelasticityofcrimewithrespecttoimprisonmentobtainedfromequation(2)is–3.37.Inotherwords,togeta10percentreductioninburglarywewouldneedtoincreasethenumberofburglarsin
Figure 5: Prison/crime elasticity as a function of offending rate
0.00
1.00
2.00
3.00
4.00
5.00
6.00
21 23 25 27 29 31 33 35 37 39
Offending rate
Ela
stic
ity(p
erce
nt)
Figure 4: Burglaries prevented by clear-up rate
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
5 6 7 8 9 10 11 12 13
Clear-up rate for burglary (per cent)
Inca
paci
tatio
nef
fect
(per
cen
t)
BUREAUOFCRIMESTATISTICSANDRESEARCH
8
prisonbyabout33.7percent.Atpresentthereareabout1,135convictedburglarsinNSWprisons.11Itfollowsthat,togettheburglaryratedownby10percentwewouldneedtoincreasethenumberofburglarsheldinprisonbyabout382.TherecurrentcostofkeepingsomeoneinprisoninNSWis$189.10dollarsperday.12Thecostofa10percentreductioninburglaryviaincapacitationwouldthereforeamounttoalittleover$26millionperannum.
Inarrivingatthisfigurewehaveassumedthattheincreaseinimprisonmentcomesaboutsolelyfromkeepingthecurrentstockofburglarsinprisonforlonger.Aswediscussedearlier,ifwechangetheproportionofburglarswhoareimprisonedwearelikelytofindourselvesimprisoningoffenderswhoseoffendingfrequencyislower.Figure5illustratesthispointbyplottingelasticity(E)asafunctionofoffendingfrequency(λ).
Itisobviousthattheelasticityofcrimewithrespecttoprisonismuchhigheratlowlevelsofoffendingfrequency.Inotherwords,iftheaverageoffendingfrequencyamongburglarsweresignificantlylowerthan38.1offencesperyear,thesizeoftheincreaseintheprisonpopulationrequiredtoproducea10percentreductioninburglarywouldbemuchhigher.If,forexample,λwere60percentofitsassumedvalue(i.e.ifλ=23),thecostofgettinga10percentreductioninburglaryviaincapacitationwouldrisetoover$43millionperannum.
dIscussIon
Thefirstpointtoemergefromtheforegoinganalysisisthat,notwithstandingoccasionalsuggestionstothecontrary,atleastsofarasburglaryisconcerned,prisondoesseemtobeaneffectivecrimecontroltool.Ourbestestimateoftheincapacitationeffectofprisononburglary(basedontheassumptionthatburglarscommitanaverageof38burglariesperyearwhenfree)is26percent.Thisestimatedoesnotappeartobeoverlysensitivetothevalueofoffendingfrequencyweassume.Ifthetruerateatwhichburglarscommitburglaryis
32offencesperyear,forexample,the
incapacitationeffectofprisonfallsto23
percent.Ifthetruerateis44offencesper
year,theincapacitationeffectrisesto28
percent.
Thesepercentageeffectsmightnot
seemlargebutinabsolutetermsan
incapacitationeffectof26percentis
equivalenttopreventingover44,700
burglariesperannum.Moreover,because
offendersgenerallycommitavarietyof
differentoffencetypes(Tarling1993,
p.120),wecanbereasonablycertain
thatimprisoningburglarsprevents
otherkindsofcrimeaswell.Itmustbe
remembered,however,thatourestimates
ofincapacitationarebasedonanumber
ofassumptions.Theseare:(1)thatthe
parametervaluesonwhichourestimate
ofincapacitationisbasedarereasonably
accurate(2)thatalloffendersruntherisk
ofbeingarrestedandincarcerated(3)the
moreoffendersweimprison,thefewer
thereareinthegeneralpopulationand
(4)thattheexperienceofimprisonment
doesnotchangetheexpectedlengthof
acriminalcareer(TR)ortherateatwhich
individualsoffend(λ).Wewillnowcritically
examineeachoftheseassumptions,in
turn.
Therearefewgroundsforconcernabout
q,SandJbecausetheyarerelatively
easytomeasure.Errorsofmeasurement
aremorelikelywithTR(residualcareer
length)orλ(offendingfrequency).TR,it
willberecalled,wasobtainedfromthe
equationTR=PA S /(1-PA),wherePAis
theprobabilitythatanoffenderremains
activeafterreleasedfromprison.The
equationisvalidiftimetore-offend
andtimeincustodyinNSWareboth
exponentiallydistributedandAppendix1
suggeststhattheyare.TheparameterPA
wasestimatedfromalarge-scalestudyof
re-offendingamongstparoleesreleased
fromprison.Therewasnoevidenceinthis
studythatratesofre-offendingwouldhave
beenhigherwithalongerfollow-upperiod.
Unlesssubstantialnumbersofparolees
returntocrimewithoutbeingre-arrested,
thenthereislittlecauseforconcernabout
TR.If,however,TRishigherthanwehave
assumed,wewillhaveunderestimatedthe
incapacitationeffectofprison.
ThevalueofλchosenforouranalysiswasbasedonSalmelainen’s(1995)studyofself-reportedoffendingamongjuvenileoffenders.Itispossiblethatthevalueofλforadultoffendersisverydifferent.Themainreasonforbelievingthisisnotthecase,isthatouralternativeestimate,obtainedusingequation(3)andbasedonoffendingbybothjuvenileandadultoffendersproducedverysimilarresults.13Itisworthnoting,however,thatifwearewronginourestimateofλwearemorelikelytohaveunderestimateditsvalue(forimprisonedoffenders)thantohaveoverestimatedit.ThisisbecausewetruncatedSalmelainen’s(1995)offendingfrequencydistributionatthe90thpercentileinordertoexcludevaluesofoffendingfrequencywedeemedtobeimplausiblyhigh.Ifthesecaseshadbeenincluded,thevalueofλwouldhavebeenconsiderablyhigher,inwhichcaseourestimateoftheincapacitationeffectofprisonwouldhavebeentoolow.
Itisimpossibletotestassumption(2)butitseemshighlyunlikelythatlargenumbersofburglarsfaceazeroriskofarrestandimprisonment.Assumption(3)ismoreproblematic,atleastinthelongrun.Thereisamarketforstolengoodsandifprisoncreatedasignificantunmetdemandforthesegoodsitispossiblethatnewthieveswouldenterthatmarket.Totheextenttowhichthishappens,ouranalysiswillhaveoverestimatedthelong-termbenefitsofincapacitatingburglars.Thevalidityofassumption(4)isdifficulttoassess.Sendingpeopletoprisonmaymakethemmorelikelytore-offendbutrehabilitationprogramsmayreducetheriskoffurtheroffending.Somearguethatthesetwoeffectscanceleachotheroutattheaggregatelevel(Cohen1983,p.10)butthereisnowayofknowingwhetherthisistrue.Note,however,thatifsendingpeopletoprisondoesmakethemmorelikelytore-offend,theincapacitationeffectoflongerprisontermswillbehigherthanourestimatessuggest.14
Thefactthatprisoniseffectiveinpreventingalargenumberofburglariesraisesthequestionofwhetherincreaseduseofimprisonmentwouldbeaneffectivewayoffurtherreducingtheburglaryrate.Ourfindingsonthis
BUREAUOFCRIMESTATISTICSANDRESEARCH
9
issue,likethoseofincapacitation
studiesinBritainandtheUnitedStates
(Cohen1978;Tarling1993),arenot
thatencouraging.Theysuggestthat
adoublingofthesentencelengthfor
burglarywouldcostanadditional$26
millionperannumbutwouldonlyreduce
theannualnumberofburglariesbyabout
eightpercentagepoints.Adoublingof
theproportionofconvictedburglars
wouldproducealargereffect(about12
percentagepoints)butonlyifthosewho
arethesubjectofournewpenalpolicy
offendasfrequentlyasthosewhoare
currentlybeingimprisoned.Givenwhat
weknowaboutthefrequencyofoffending
amongstburglarswhodonotcurrently
receiveaprisonsentence,thisseems
highlyunlikely.
Itmightbeobjectedthat$26million
isasmallpricetopaywhenweighed
againstthecostofburglary.Theannual
burglaryinsuranceclaiminNewSouth
Walesissomewherebetween$3,500and
$3,800.15Ifwetakethelowerofthesetwo
figuresandmultiplyitbytheestimated
numberofburglariespreventedasresult
ofimprisonmentwearriveatafigureof
$156millionasthenetdollarsavings
obtainedasaresultofimprisoning1,135
burglars.Thisisnearlytwicetheannual
costofkeeping1,135burglarsinprison
inNewSouthWales.Ofcourse,the
averagecostofburglariesnotreported
topolicemaybesubstantiallylowerthan
theaveragecostofburglariesthatare
reported.Howeverevenifthetruecost
ofeachburglarywereonlyhalfthefirst
amountcitedabove,therecurrentcost
ofimprisoningburglarswouldstillbeon
parwiththefinancialcostofburglary.16
Onthesurface,then,itwouldseemthat
thereisacompellingcaseforgreateruse
ofimprisonmenttocontrolburglary.
Whenassessingthemarginalbenefitsof
higherimprisonmentrates,however,the
relevantissueisnotwhetherprisoncosts
lessmoneythanitsavesbutwhetherit
isthemostcost-effectivewayofbringing
crimedown.Giventhecurrentstateof
knowledgewecannotevenbeginto
answerthisquestion.Therearepolicing
strategies(e.g.targetedpatrolsatcrime
hotspots,weaponsconfiscation)and
criminaljusticeprograms(e.g.coercedtreatment,cognitivebehaviouraltherapy,post-releasesupport)thathavebeenshowntobeeffectiveinreducingcrimeandre-offending(Shermanetal.2002).Anyoneoftheseprogramsandstrategiesmightbemorecost-effectivethanprisonincontrollingcrime.Inthevastmajorityofcases,however,wehavenoinformationwhatsoeveronthecostoftheseprograms,letaloneonwhichprogramsproducethegreatestreturnoninvestment(Welsh&Farrington2000).17
Therearethreeotherimportantconsiderationsthatalsoneedtobeborneinmindwhenconsideringwhethertoincreaseimprisonmentratestoreducetheburglaryrate.Firstly,suddenincreasesinpenaltyseverityaresometimesaccompaniedbyareductionintheproportionofdefendantswillingtopleadguilty,withtheresultthatfewerdefendantsendupconvictedandmoreofthosewhoareconvictedendup(asaresultofpleabargaining)convictedonlessercharges(Cohen&Tonry1983;Ross&Foley1987).Intermsofthemodelexaminedhere,thiswouldmeanthatanygaininincapacitationachievedbychangingJorS,maybenullifiedorpartiallyoffsetbyareductioninq.
Secondly,evenifprisondoesexertabeneficialshort-termeffect,havingaprisonrecordsubstantiallyreducestheemploymentandearningsprospectsofoffenders(HaganandDinovitzer1999).Thismayprolongtheperiodofinvolvementincrime.Thebenefits,intermsofcrimecontrolthataccruefromputtingmoreoffendersinprisonthereforeneedtobecarefullyweighedagainstanylong-termcriminogeniceffects.GiventheinordinatelyhighlevelsofIndigenousoverrepresentationinthejusticesystem(Weatherburn,Lind&Hua2003),thisisanissueofparticularimportancewhereIndigenousoffendersareconcerned.
Thirdly,whiletheeffectivenessofprisonincontrollingcrimeisanimportantconsiderationinframingpenalpolicy,itisnotbyanymeanstheonlyconsideration.Theuseofprisonasacrimecontroltoolraisesimportantethicalissues,particularlywhereitisbeingusedtopreventfuture
offendingratherthantopunishoffendersforpastoffences.Aswellasbeingeffective,thepenaltiesimposedbythecourtshavetobefairandjust.Wemaybeabletosubstantiallyreduceburglarybymakinggreateruseofimprisonmentbutthereisnoguaranteethatthepenalpoliciesrequiredtoachievethisoutcomewillbeacceptabletothegeneralcommunity,especiallyiftheyinvolvemuchhigherlevelsofimprisonmentforjuvenileoffendersandothervulnerablegroupsinthecommunity.
Giventheenthusiasmwithwhichsomemediacommentatorsgreetanysuggestionthattougherpenaltiesareeffectiveinreducingcrime,itmightbeworthsoundinganoteofcautionagainstanytendencytoassumethatbecauseprisonexertsasubstantialpreventativeeffectonburglary,itmustexertasubstantialpreventativeeffectonotherkindsofcrimeaswell.Aswehavealreadyseen,theincapacitationeffectofprisondependsuponalargenumberoffactors.Thesefactorsmayandprobablydovarysubstantiallyfromonegroupofoffenderstoanother.Theonlywaytogaugetheincapacitationeffectofprisononotherkindsofcrime,then,istorepeattheanalysisconductedhereforotherkindsofcrime.
Thequalificationssurroundingourfindingsandthelimitedscopeofourstudymaybeviewedbysomeaslimitingitsutilityingaugingthevalueofprisonasacrimecontroltool.Itwouldindeedbeunwisetobasefuturedecisionsaboutpenalpolicysolelyonthebasisoftheresultsreportedhere.Iftheydemonstratenothingelse,however,ourfindingsshowthatprisonshouldneitherbedismissedasirrelevanttocrimecontrolnortreatedasapanacea.Theevidencethatprisonstopsalotofcrimeisverystrong.Thecost-effectivenessoffurtherinvestmentinprisonrelativetootheroptionsforbringingdowncrime,however,isveryunclear,notonlyforburglarybutforallotheroffencesaswell.Attheriskofstatingtheobvious,thereisapressingneedforfurtherAustralianresearchintothecost-effectivenessofprisonanditsalternativesinpreventingandcontrolling
crime.
BUREAUOFCRIMESTATISTICSANDRESEARCH
10
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acknowledgement
WewouldliketothankMrFrankMorgan,
AssociateProfessorPaulMazerolle,
ProfessorJanetChanandDrToniMakkai
forprovidingvaluablecriticalfeedbackon
anearlierdraftofthisbulletin.Wewould
alsoliketothankTracyPaintingand
SuzannePoyntonfortheircarefulediting
andFlorenceSinfordesktoppublishing.
notes
Thefallinimprisonment,unfortunately,
wasaccompaniedbyadeclineinclear-
upratesforrobbery,makingitdifficultto
disentanglethetwoeffects.
Thisadjustmentisconventionallymade
(seeBlumsteinetal.1986,p.60),however,
PaulMazerollehaspointedoutthat
incarceratingoneoffenderfromagroup
maynotmateriallyaffecttherateatwhich
thegroupoffends.
1.
2.
StevensonandForsythe(1998)report
medianoffendingfrequencyratherthan
meanoffendingfrequency.Whenweused
theirrawdatatocalculateanoverallmean
offendingfrequencythecalculationrevealed
anaveragerateof407offencesperyear,
animplausiblyhighfigure.Ifacceptedit
wouldimplythat,ifalltheburglarscurrently
heldinprisonweresetfree,thenumberof
burglarieswouldrisetomorethanfourand
ahalftimesitscurrentlevel.Theproblem,it
seemslieswiththequestionStevensonand
Forsytheusedtoobtaininformationabout
offendingfrequency.WhereasSalmelainen
simplyaskedrespondentshowmany
burglariestheyhaddoneinthesixmonths
leadinguptothearrestthatresultedintheir
incarceration.StevensonandForsythefirst
askedtheirrespondentstoindicatewhether
theydo‘break,enterandsteals’:(a)‘every
dayoralmosteveryday’,(b)‘severaltimes
aweek’,(c)‘everyweekoralmostevery
week’(d)‘lessthanonceaweek’,(e)‘less
thanonceamonth’or(f)‘other’.Ifthey
answeredintheaffirmativeto(a)theywere
askedhowmanyoffencesperdaythey
hadcommittedperday.Iftheyansweredin
theaffirmativeto(b)theywereaskedhow
manyoffencestheycommittedperweek.
Iftheyansweredintheaffirmativeto(c)or
(d)theywereaskedhowmanyoffences
theycommittedpermonth.Iftheyanswered
‘other’theywereaskedhowmanyoffences
theycommittedintheprevioussixmonths.
Alargepercentageofthosewhoprovided
informationgavegrosslyimplausible
answers(severalinvolvedclaimsof
overathousandofburglariesperyear).
Manyrespondentsalsoseemedtogive
answersthatsimplycorrespondedwiththe
beginningofthereferenceperiod.Inthe
circumstanceswejudgeditunwisetorely
ondataonoffendingfrequencytakenfrom
theStevensonandForsythestudy.
Weincludepeoplechargedwithburglary
butconvictedofsomeotheroffenceon
thegroundsthatburglarsconvictedofand
senttoprisonfornon-burglaryoffences
cannotcommitburglary.Thisassumes,of
course,thatanyonechargedwithburglary
committedtheoffence.
Ifseveralminimumtermsareimposedasa
consequenceofbeingconvictedofseveral
offences,wetaketheminimumtermforthe
principaloffence(i.e.thelongestminimum
termtowhichtheoffenderissubject).
3.
4.
5.
Thelowervalueobtainedinthisstudyis
probablyareflectionofthefact(a)that
averagesentencelengthsforburglaryhave
beenincreasing(Moffatt,Weatherburn&
Donnelly2005)and(b)thatsomeofthose
includedintheminimumtermestimatesfor
burglarshadalsobeenconvictedofmore
seriousoffencesandthereforehadlonger
minimumterms.
Thisfigureaccordsverywellwiththat
obtainedbyTarling(1993)forPAinhis
studyofincapacitationinBritain.
Theadjustmentisforco-offendingeffects
(seemethodsection)
Itisreassuringtonotethatthisisvery
closetothefigureyouobtainwhenyou
multiplythenumberofimprisonedburglars
(1135)bytheassumedvalueofλ(38)
bytheaveragesentencelengthinyears
(1.02).
Thisistheearliestdatafromwhichwecan
trackanindividual’scriminalrecord.
Personalcommunication:KyleighHeggie,
CorporateResearchandEvaluation,NSW
DepartmentofCorrectiveServices.
Factsandfigures:CorporateResearch,
Evaluation&Statistics,CorrectiveServices
August2005.
Theseemingstabilityofλsuggeststhat
thecurrentheroinshortage(whichbegan
afterSalmelainenconductedherstudy)
hasnothadmucheffectontheoffending
frequencyofthosewhoremainedinvolved
inburglary.WeareindebtedtoDrToni
Makkai(DirectoroftheAustralianInstitute
ofCriminology)forraisingthisissuewith
us.
Asthemeannumberofoffencesincreases
ortheaveragelengthofacriminalcareer
lengthincreases,theproportionofcrime
avertedthroughlongerprisontermsalso
increases.
DatakindlysuppliedbyaseniorAustralian
insuranceindustryexecutivewhowishesto
remainanonymous.Thesmallerestimate
relatestohouseholdinsuranceclaims.
Thelargerrelatestoburglaryclaimsby
businesspolicy-holders.
Therecurrentcostofkeeping1,135
burglarsinprisonisapproximately$78.3
millionperannum.Thisfigureisobtained
bymultiplyingthenumberofimprisoned
burglars(1,135)bythedailycostof
imprisonment($189.10)by365.
ForanexceptionseeLindetal.2002.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
BUREAUOFCRIMESTATISTICSANDRESEARCH
12
aPPendIx
NSWBureauofCrimeStatisticsandResearch-Level8,StJamesCentre,111ElizabethStreet,Sydney2000
[email protected]•www.lawlink.nsw.gov.au/bocsar•Ph:(02)92319190•Fax:(02)92319187
ISSN1030-1046•ISBN0731326768
Figures6and7showtheobservedandpredicteddistributionsoftimetore-offend(TTR)andprisontimeserved(PT)ontheassumptionthatthetruedistributionsineachcaseareexponential.Thesamplemeanineachcasehasbeenusedtocalculatethepredicteddistribution.
TheexponentialisamuchbetterfittotheTTRdistributionthantothePTdistribution,probablybecauseofatendencyonthepartofjudgesandmagistratestoimposetermsofimprisonmentthataremultiplesofthreemonths.ThepredictedsurvivaltimesseemtodeviateatbothendsofeachofthedistributionsbutKolmogorov-Smirnofftestsindicate(p(TTR)=0.22;p(PT)=0.87)thatthedifferencesarewellwithintherealmofchance.CumulativeprobabilityplotsshowgoodfittotheexponentialforbothTTRandPT,withtheexceptionthatlowvaluesofPTtendtodeviatemorefromtheexpecteddistribution.
Figure 6: Time to reoffend (TTR) for paroleesFit to exponential distribution for frequencies observed over 50 day intervals
Time interval in days
0102030405060708090
100
Observed Expected
Figure 7: Prison time served (PT) Fit to exponential distribution for frequencies observed over 100 day intervals
0
20
40
60
80
100
120
Time interval in days
Observed Expected