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.
One of the things he didn’t do when giving the numbers a greater examination was standardise for age. Māori are, on average, younger than non-Māori. Committing the sort of crimes that get you caught and sent to prison is more common among young people, so even if there were no bias in the justice system you’d expect a higher proportion of Māori out of those in prison than out of the general population. People noticed the lack of age adjustment. There was Twitter.
The problem is data availability: prison population by age and ethnicity is not a published table. So, how much will the age thing matter? A priori it could matter a lot. It does for some health questions. On the other hand, you’d expect a sociologist who wrote
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:
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.