And here's where it gets really interesting:
You need to be careful when crafting OIA questions around this sort of comparison, and I am relying on the Ministry of Justice to have correctly understood my intention. It is not enough to compare the number of convictions before and after the law change. Almost a third of convictions for “strike” offences since three strikes was enacted haven’t attracted first warnings because they relate to offending that occurred before the law change. You need to exclude similar offending in the comparison.
Between 1 June 2005 and 31 May 2010, 6809 people received convictions for strike offences that occurred between 1 June 2005 and 31 May 2010.
Between 1 June 2010 and 31 May 2015, 5422 people received convictions for strike offences that occurred between 1 June 2010 and 31 May 2015. So strike crime is down around 20% since three strikes came into effect. Claiming cause and effect over something like that is the type of intractable debate that you get into over the effect of longer prison sentences. But what we are looking at is not the general deterrent effect of three strikes (fear of punishment in the public at large), but specific deterrence: fear of punishment by those who have a conviction for strike offending who have been personally warned by a judge that further strike offending is treated very seriously. And that is where we can check the comparison between the five years before three strikes and the five years after it.
We know there were 81 second strikes in the first five years of three strikes. These are people who have been convicted for committing a strike offence after the law came into force, and subsequent to that conviction, been convicted of a further strike offence, itself committed after their earlier conviction occurred. The pre-strike comparison therefore needs to be people convicted of an offence committed after 1 June 2005 (but before 31 May 2010), who were then convicted before 31 May 2010 of a further offence committed after that conviction. And it turn out that that number is a lot higher. Had the three strikes law been in place on 1 June 2005, the following five years would have seen 256 offenders receive second strikes.
Now, strike crime is down in general, but the ~20% fall in strike offending is dwarfed by the ~62% fall in strike recidivism.
It looks like everything is there for a proper full difference-in-difference analysis, but ocular least squares here seems to be pretty strong.
Because of the lack of retrospectivity in our three strikes law, two people convicted on the same day, in respect of the same charge can have different strike consequences: someone convicted for offending that occurred after the law came into force receives a strike warning, but someone convicted of an offence committed before the law was enacted receives no warning.
A comparison between these two groups may help confirm or quash the alternative hypothesis that some change in treatment is the cause of the substantial reduction in strike recidivism.
In the first 4 years and 7 months of three strikes (curse you tier one statistics!), 2437 people have been convicted of strike offending that did not result in a strike warning, and of those, 360 had subsequently earned a first warning for an offence committed after that conviction. That’s a strike recidivism rate over 1000% higher among those who didn’t receive a warning than those who did. Of course, this direct comparison is misleading, as the post-strike convictions for pre-strike offending will be front-loaded, occurring on average much earlier in the ~5 year period since three strikes was enacted, and thus allowing more time for strike-level recidivism to occur. However, it remains useful, as it provides evidence to negate the alternative explanation for the pre-strike/post-strike comparison of much improved recidivism treatment.
And that is what we are left with: in the first five years of three strikes, there were 81 second strike convictions. In the five year before three strikes, there would have been 256.
81 second strikes seemed low. Now we know it is.
A full set-up would compare the year t+1, t+2, t+3 and so on recidivism rates of those convicted of a first strike offence for offending that happened before and after the regime and of other offences before and after the regime.
Were I at Canterbury, I'd be begging Graeme for his data as basis for setting a Masters thesis and putting in the OIA requests for the finer-grained data that could let it happen.
UPDATE: Graeme has found that his early and later data was not comparable, and so has retracted this analysis.