Further data on the Peterborough Social Impact Bond

The Office of National Statistics provided further data on Peterborough at the end of July, this time on the complete first cohort of 1,000 prisoners.

While this is largely confirmatory information, the Ministry of Justice found a closer matching baseline, by focusing on local prisons rather than all national prisons. This responds to the concern that Peterborough may be hard to emulate or unrepresentative as it is local and therefore returns more of its prisoners to the local area.

The updated data looks like this:

Peterborough (and national equivalent) interim re-conviction figures of cohort 1 with a 6 month re-conviction period


National local prisons

Discharge Period

Cohort size











 Sep06- Jun08






 Sep07- Jun09






 Sep08- Jun10






 Sep10- Jun12






Binary: Reconviction rate over six months
Frequency: Frequency of reconviction events per 100 offenders within six months

A few topics to cover:

– Is this a better baseline and therefore does it give us greater confidence in the effect that Peterborough is having?

– Is this data good, or mixed as some have reported?

1. Is this a better baseline?
It should be, as it better matches the Peterborough cohort. As an experiment, I thought I would put together similar graphs to the ones before and compare them.

Data to March, with National baseline
Rebased reoffending data

Data to June with National local baselineRebased reoffending 2

And now the relative change graphs

Data to March, with National baseline
Peterborough relative to national

Data to June with National local baselinePeterborough relative to national2

What this shows visually is that the new baseline appears to be a better fit. Movements in the baseline prior to the intervention are closer to the movements in the Peterborough cohort, in other words the baseline appears to explain more of the movement in the Peterborough data. So it gives us greater confidence that we are seeing an intervention effect.

It also gives us a degree of greater confidence that we will get paid. The previous data ended with Peterborough’s frequency number equalling the national average. This one ends with Peterborough at least improving upon it. The propensity score matching process should bring out a comparison cohort that is even more similar, but of course we still haven’t tried it.

So, is it time to pop open the champagne and celebrate? Not yet. This is good news, but it is still only on six month data. We will be measured on whether we reduce offending over twelve months. What we can say is that our intervention appears to at least delay reoffending behaviour.

We should also say, this is only the first cohort of the first Social Impact Bond. It is incredibly early days so drawing significant conclusions at this stage is premature. On the other hand, we are learning and developing all the time, so the fact that we see a significant impact on such an early group is clearly exciting.

2. So is this data good, or mixed as some have reported?

We are cautious, because this is early days and early data. It isn’t a randomised control trial, sure. Nor is it the formal comparison cohort that will be developed for payment purposes using propensity score matching. But this is very positive data, on the best available information.

In the first set of results, which were also good, one of the caveats people put forward was that the Peterborough frequency was now only the national average. On this closer baseline this is no longer the case.

Another concern was that the jump in re-offending frequency in the national data should be treated with caution. I understand that, and see the potential for regression to the mean, but comparison with national data is more precise than looking at a comparison with historical data. Thus the 20% relative decline is the more useful figure than the 8% decline against historical figures, particularly given the strong correlation between the local prison data and the Peterborough data historically.

It is important to draw a distinction between responding with caution, on the basis of the caveats outlined above, and saying that results are “mixed” as we have seen in a few quarters. They’re not mixed, they’re surprisingly strong – but early and indicative at this stage.