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.

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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).

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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

bcsr@agd.nsw.gov.au•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