April 28, 2016

Māori imprisonment statistics: not just age

Jarrod Gilbert had a piece in the Herald about prisons

Fifty per cent of the prison population is Maori. It’s a fact regularly cited in official documents, and from time to time it garners attention in the media. Given they make up 15 per cent of the population, it’s immediately clear that Maori incarceration is highly disproportionate, but it’s not until the numbers are given a greater examination that a more accurate perspective emerges.

The numbers seem dystopian, yet they very much reflect the realities of many Maori families and neighbourhoods.

to know what he was talking about, qualitatively. I mean, this isn’t David Brooks.

It turns out that while you can’t easily get data on ethnicity by age in the prison population, you can get data on age, and that this is enough to get a good idea of what’s going on, using what epidemiologists call “indirect standardisation”.

Actually, you can’t even easily get data on age, but you can get a graph of age:
ps_ages_3_16

and I resorted to software that reconstructs the numbers.

Next, I downloaded Māori population estimates by age and total population estimates by age from StatsNZ, for ages 15-84.  The definition of Māori won’t be exactly the same as in Dr Gilbert’s data. Also, the age groups aren’t quite right because we’d really like the age when the offence happened, not the current age.  The data still should be good enough to see how big the age bias is. In these age groups, 13.2% of the population is Māori by the StatsNZ population estimate definition.

We know what proportion of the prison population is in each age group, and we know what the population proportion of Māori is in each age group, so we can combine these to get the expected proportion of Māori in the prison population accounting for age differences. It’s 14.5%.  Now, 14.5% is higher than 13.2%, so the age-adjustment does make a difference, and in the expected direction, just not a very big difference.

We can also see what happens if we use the Māori population proportion from the next-younger five-year group, to allow for offences being committed further in the past. The expected proportion is then 15.3%, which again is higher than 13.2%, but not by very much. Accounting for age, it looks as though Māori are still more than three times as likely to be in prison as non-Māori.

You might then say there are lots of other variables to be looked at. But age is special.  If it turned out that Māori incarceration rates could be explained by poverty, that wouldn’t mean their treatment by society was fair, it would suggest that poverty was how it was unfair. If the rates could be explained by education, that wouldn’t mean their treatment by society was fair; it would suggest education was how it was unfair. But if the rates could be explained by age, that would suggest the system was fair. They can’t be.

avatar

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 »

Comments

  • avatar
    Jamie Whyte

    “If the rates [of incarceration] could be explained by education, that wouldn’t mean [Maori’s] treatment by society was fair; it would suggest education was how it was unfair.”

    How would it suggest this? How can you see fairness and its nature in statistics?

    What do you mean by “treatment by society”? Why is Maori education caused by society but not the average age of Maori? After all, the educational achievement of populations and their average age are correlated.

    1 year ago

  • avatar
    Thomas Lumley

    Ok. Put it this way:

    1. There is a correlation between ethnicity and imprisonment

    2. It is not an outlandish hypothesis that this correlation is caused by current or past discrimination.

    3. However, if the correlation vanished when conditioning on age this would be strong evidence against the hypothesis and in favour of confounding by age as the cause.

    4. Having the correlation vanish when conditioning on socio-economic variable is not similarly strong evidence because these variables could easily themselves be affected by current or past discrimination. All it would do is make the hypothesis more specific about how the effects of current or past discrimination were mediated.

    1 year ago

  • avatar
    Derrilin Webb

    Good discussion on the age demographics, however I think that the general ‘message’ of the article can also be questioned.

    Specifically their hypothesis (is as you’ve stated) – There is a correlation between ethnicity and imprisonment (the subtext is that it’s being caused by racism).

    So they state 51% of the prison population is Maori, whereas 14.9% of NZ’s population identifies as such.

    They haven’t stated that 3.4% of the prison population is Asian, whereas 11.8% of NZ’s population identifies as such.

    And 12% of the prison population is Pacific peoples, whereas 7.4% of NZ’s population identifies as such.

    If they assume that racism accounts for the over representation of Maori, are they also claiming that the ‘system’ is positively racist towards Asians and lets them off crimes they are committing?

    In addition, there is also the gender demographics where 93.3% of the prison population is Male, whereas 48.7% of NZ’s population identifies as such. And I assume that they are not claiming that the ‘system’ is sexist towards Men, but rather Men are more likely to commit prisonable offenses.

    While there could be racist elements in the ‘system’ I don’t think that assuming that if there is not a proportional representation across each demographic then that is a sign of racism or sexism is a sound position.

    1 year ago

  • avatar
    Megan Pledger

    When 13% of the pop is Maori and 50% of the prison pop is Maori than something is going on that isn’t good.

    ~~
    I recall hearing that the number of people on home detention is about 1/3 of the imprisoned population. It could be that more Maori don’t have a suitable place in which to serve home detention so it skews the stats.

    1 year ago

  • avatar
    steve curtis

    Home detention is only for sentences of 2 years or less.
    Another factor for the high Maori imprisonment is gang affiliation. Look at gang membership by ethnicity and you will find it too is skewed.

    1 year ago

  • avatar
    Megan Pledger

    I’m not sure what your point is.

    Although the data is from 2006, 87% of sentences were for two years or less.

    Table 4.3
    http://www.justice.govt.nz/publications/publications-archived/2008/conviction-and-sentencing-of-offenders-in-new-zealand-1997-to-2006/4-custodial-sentences-and-remands

    If your family is living in poverty than it actually makes financial sense to go to jail rather than do the time at home. Being at home costs money for food, electricity, water rates etc. In jail all that is paid for.

    1 year ago

    • avatar
      steve curtis

      Your data maybe but the data here covers all prisoners and is up to date.

      Is 10 year old data that useful when considering something fairly new like home detention.

      1 year ago

    • avatar
      Thomas Lumley

      That’s proportion of sentences, not proportion of people in prison, though. The average length of sentence for a person in prison will be longer, because of length-biased sampling.

      1 year ago

  • avatar
    Matt Haynes

    slightly off topic but i’d thought I’d ask here. Theres talk of Maori being imprisoned more often than non-maori for the same crime, I’m seeing it a lot lately but I’m not convinced. Are prior convictions being taken into account? If Maori are generally committing more crime than non-pakeha, when they do commit a crime that borders on a prison sentence, won’t the previous convictions be the difference for the judge in handing down a jail term….or not??
    Hence an appearance of maori getting sent to jail more often than non-maori for the same offence?

    1 year ago

    • avatar
      Thomas Lumley

      It’s not as simple as that: if Māori are more likely to be arrested, charged, convicted, or imprisoned for the same crime they will then acquire worse criminal records for the next time around. So it’s hard to tell.

      One independent data point is drug use, where there is survey data that doesn’t come from the judicial system. Māori are over-represented in convictions for cannabis use/possession () but the survey data on rates of cannabis use show a much smaller difference.

      Also, unlike age as an explanation, criminal history wouldn’t mean this wasn’t a problem, it would just change what sort of policies might help.

      1 year ago

      • avatar
        Matt Haynes

        Yes, now that I start looking into into it I’d have to agree – it really isn’t that easy!

        I found this really helpful with some great stats around ethnicity and imprisonment. Unfortunately they didn’t investigate prior convictions in regards to sentencing outcomes

        http://www.corrections.govt.nz/resources/research_and_statistics/over-representation-of-maori-in-the-criminal-justice-system.html

        I did find this statement amongst the report –

        “Of those who pleaded guilty,
        considerably fewer Māori were first offenders (17%, vs. 29% of all non-Māori). Also, importantly, more Māori (37%) than non-Māori (31%) were apprehended for offences of medium to high seriousness.”

        Which certainly suggests their prior offending is influencing their sentencing – but we don’t know by how much.

        In regards the the parole system and chance of re-offending things get really interesting –

        “Findings from this recent re-calibration of RoC*RoI have important implications for
        the current question of whether disproportionate representation of Māori within
        criminal justice statistics is reflective of bias. On the basis of the RoC*RoI data it can
        reasonably be deduced that, following a re-conviction, the likelihood that an offender
        will be sentenced to a term of imprisonment, rather than a community-based sentence, is not significantly affected by the offender’s ethnicity. That is, in the case of offenders of different ethnicity, where all else is equal (for example, seriousness of current offence, number of previous convictions, number of previous prison terms,etc), equivalent sentencing outcomes can be expected.”

        Which is pleasing to see in that regard.

        Looks like there’s some work to do around bias in apprehension, prosecution, home detention and sentencing. Although this report seems to think this is of relatively minor importance to the sheer numbers of Maori being imprisoned. Definitely worth a read

        1 year ago

  • avatar

    It is likely that age is ONE factor – but not the ONLY or BIGGEST factor.

    Rather, I suspect it is a combination of a number of factors, which include age, income (or a lack thereof), plus ethnicity, plus bias/racism.

    10 months ago