September 30, 2015

Three strikes: some evidence (updated)

Update: the data Graeme Edgeler was given didn’t mean what he (reasonably) thought they meant and this analysis is no longer operative. There isn’t good evidence that the law has any substantial beneficial effect.  See Nikki Macdonald’s story at Stuff and Graeme’s own post at Public Address.

The usual objection to a “three-strikes” law imposing life sentences without parole, in addition to the objections against severe mandatory minimums, is

  • It doesn’t work; or
  • It doesn’t work well enough given the injustice involved; or
  • There isn’t good enough evidence that it works well enough given the potential for injustice involved.

New Zealand’s version of the law is much less bad than the US versions, but there are still both real problems, and theoretical problems (robbery and aggravated burglary both include crimes of a wide range of severity).

Graeme Edgeler (who is not an enthusiast for the law) has a post at Public Address arguing that there is, at least, evidence of a reduction in subsequent offending by people who receive a first-strike warning, based a mixture of published data and OIA requests.

Here’s his data in tabular form, showing second convictions for offences that would qualify under the three-strikes law. The red cell is ‘first strike’ convictions, the other rows did not count as strikes because the law isn’t retrospective.

Offence Conviction Number Second conviction Number
7/05-6/10 7/05-6/10 6809 7/05-6/10 256
Before 7/10 7/10-6/15 2437 7/10-1/15 300
7/10-6/15 7/10-6/15 5422 7/10-6/15 81


The first and last rows are directly comparable five-year periods. Offences that now qualify as ‘strikes’ are down 20% in the last five-year period; second convictions are down a further 62%. Data in the middle row isn’t as comparable, but there is at least no apparent support for a general reduction in reoffending in the last five-year period.

The overall 20% decrease could easily be explained as part of the long-term trends in crime, but the extra decrease in second-strike offences can’t be.  It’s also much larger than could be expected from random variation. The law isn’t keeping violent criminals off the streets, but it does seem to be deterring second offences.

Reasonable people could still oppose the three-strikes law (and Graeme does) but unless we have testable alternative explanations for the large, selective decrease, we should probably be looking at arguments that the law is wrong in principle, not that it’s ineffective.



Thomas Lumley (@tslumley) is Professor of Biostatistics at the University of Auckland. His research interests include semiparametric models, survey sampling, statistical computing, foundations of statistics, and whatever methodological problems his medical collaborators come up with. He also blogs at Biased and Inefficient See all posts by Thomas Lumley »


  • avatar
    Megan Pledger

    The strike laws give the prosecutors greater leverage to make people plead guilty to lesser charges. The upside for the prosecution is that it saves a lot of money to avoid a jury trial and the person doesn’t get off.

    It would be interesting to look at the number of charges for first and second stroke offenses rather than convictions in both periods. Are charges decreasing as well as convictions or just convictions?

    And it would be interesting to know the length of time in prison for first strike offenses in both periods. Maybe there are less second strike offenses in the second period because people were spending more time in jail from the first strike offense and so haven’t got around to committing a second strike offense yet.

    2 years ago

    • avatar
      Thomas Lumley

      The length of time for first-strike offences wasn’t changed by the three-strike laws (unlike second offences, where it affects parole, and third offences), so that shouldn’t have changed.

      Maybe it’s plea bargaining, but that’s a big and quite selective difference. If it is plea bargaining, then all prosecutors will have to *know* that’s the explanation, so a bit more OIA could clear it up.

      2 years ago

      • avatar
        Megan Pledger

        Actually, time served might be better than sentence length because IIRC there has been changes around parole. Also, in the first period, the jails were full to the brim and now they have more beds which effects parole and early release.

        The other thing might be that people doing second strike type crime might take more care to avoid being caught i.e. it’s not making fewer criminals but sneakier criminals.

        2 years ago

  • avatar
    David Garrett

    Megan, It is very difficult to find out exactly how many first strikers are currently “on the street” due to privacy reasons – supposedly – and the fact that the Corrections Dept. doesn’t keep these exact stats.

    From what we are able to work out, about 65% of the 5400 odd first strikers are currently at liberty. This is no surprise, given the lenient sentences Judges impose, and the Orwellian Sentencing Act 2002, which means no sentence – other than second or third strike sentences – handed down in New Zealand actually mean what they say.

    For example, a three or four year sentence becomes 18 months or two years respectively, and any sentence under two years is automatically cut in half.

    2 years ago

  • avatar

    Noting here that my analysis has been withdrawn, as the data were not comparable.

    10 months ago