In 2010, the world’s first Social Impact Bond (SIB) was launched at Peterborough Prison. It was used to fund an intervention – ‘The One Service’ – aimed at reducing the reoffending among prisoners discharged after serving a sentence of less than 12 months. Under the terms of the SIB, investors are paid according to how successful the One Service is in reducing reconvictions. If a minimum threshold of a 7.5% reduction in reconviction events is reached across the pilot, payment is triggered. Additionally, there is an option to trigger an early payment if a 10% reduction is noted in the number of reconviction events in individual cohorts.
A propensity score matching (PSM) approach was used to estimate the impact. For cohort 1, the impact was estimated, by a previous team of independent assessors, to be a reduction in reconviction events of 8.4% (Jolliffe and Hedderman, 2014).
This report reviews the PSM approach, prompted in part by the desire to understand the reasons behind the differences in reconviction rates between prisoners discharged from HMP Peterborough and prisoners discharged from other prisons. Its primary aim is to identify whether there is a need to revise the approach taken before it is applied to cohort 2 (and the final cohort - the weighted mean of cohort 1 and cohort 2). To do this, various amendments to the methodology were explored.
It was not possible to replicate the results of Jolliffe and Hedderman (2014). This should perhaps be no surprise given the difficulties often encountered with replication attempts. However, the difference between the replication result and the Jolliffe and Hedderman (2014) result was not statistically significant.
Since this review did not identify any clear improvement in the matching process, the conclusion is that the cohort 1 approach be maintained. We also recommend an adjustment to the sample selection in cohort 2. It is important to note that this recommendation is based on the analysis of cohort 1 data and is not informed by cohort 2 reoffending data
