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 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). Anders and Dorsett (2017a) reviewed 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. They were unable 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.
Following their review, Anders and Dorsett (2017a) recommended that the cohort 1 methodology approach be maintained for cohort 2. They did recommend a change to the sample definition for cohort 2. This was adopted for cohort 2. It is important to note that matching was performed using a dataset that excluded reoffending data.
In order to learn more from the evaluation, a separate analysis examined the sensitivity of the estimated
impacts to this change in sample definition. This report presents the findings of the sensitivity analysis. The main result is that estimated impacts can vary according to the definition of the sample. However, the differences between the estimates are not statistically significant
