Yes, this is just one study, yes it seems to have been picked out for publicity by the Cons, yes it only looks at the effects on a specific individual of past punishment directed at them, more relevant to a 'short sharp shock' doctrine than a more general conception of prudential deterrence. But does it indeed show that longer prison sentences reduce reoffending - or in four-legs-good terms, that 'prison works'? Not really.
From the study, produced under the rubric of the DoJ's Compendium of reoffending statistics and analysis
Those sentenced to 1 to 2 years in custody had lower re-offending rates than those given sentences of less than 12 months – the difference in proven re-offending rates was 4.4 percentage points in 2008.
Custodial sentences of less than twelve months were less effective at reducing re-offending than both community orders and suspended sentence orders – between 5 and 9 percentage points in 2008. This reinforces the finding in the 2010 Compendium which was only based on 2007 data. The findings were similar for both community orders and suspended sentence orders.
The findings are not conclusive on whether the deterrent effect of longer custodial sentences is effective at reducing re-offending. Despite higher re-offending rates, offenders receiving sentences of less than 12 months do not have access to offender management programmes and are not subject to supervision by the Probation Service upon release. This latter factor is also likely to explain some of the difference between community sentences/suspended sentence orders and short prison sentences. However, the true impact of offender management programmes and Probation supervision cannot be reliably established using current Ministry of Justice administrative data.
I won't try to assess the multiplicity of considerations (e.g. small samples, arcane statuistical disputes) that may mean the study is of little or no value in showing anything. So, taking the report on its own terms:
- Sentences under one year are less effective at reducing reoffending than non-custodial sentences.
- Sentences under one year, which are not accompanied by offender management programmes and probation service supervision, are less effective than sentences between 1 and 2 years, which are.
From this we might infer that prison makes things worse, but supervision etc. is so effective that it more than compensates for that.
This doesn't address the matter of the comparison of 1-2 year sentences with 2-4 years, though. The report seems to show that the longer sentence is correlated with a lower reoffending rate (during the year after release). I'm not trying to rustle up some comprehensive rebuttal here - a study is a study, there is a large literature on these questions, and for all I know longer sentences in this range do have some effect on reoffending (how that happens is another question - there are many factors that might be involved, some of which might be achieveable without custodial sentences). I will permit myslef to say that longer stences fior the same offcne might reflect other factors which could in turn lead to closer supeervision in that post-release year - i.e. there might be an informal or less clearly-documented difference of the same kind as that already addressed.
But no further. Let the last word go to the report itself:
The findings from this paper are not conclusive on whether the deterrent effect of longer custodial sentences is effective at reducing re-offending.