Sample size requirements are totally counter intuitive. I remember my statistics professor mathematically demonstrating to us the relationship between sample size and population size. Our minds were boggled.
For your personal enbogglement, I’ve charted that relationship here. It shows the sample size needed to get a 1 per cent confidence interval with a 99 per cent confidence. The salient point is the flattening out at the top.
It is intuitive that if you’re enquiring about a big population, you need a big sample. Intuitive, but wrong.
Once you’re surveying a population of more than 500,000, there’s scarcely any need to increase your sample size. Sampling more than 16,000 people out of a large population means adding very little value. This is good news. It means our society’s reliance on survey data – for everything from who should be PM to how peanut butter should taste – is efficient.
So why do we run censuses? Partly because the Census Act 1905 compels us to, every five years. Partly because we always have, and partly because the UN encourages us to.
The census offers value, no doubt. I use the data often. It makes sure no community is left out. You can use the census portal to dive down into detail on where you live and get accurate data.
But does the census get localised data effectively?
If you’re interested in small, remote, unusual communities that you will never otherwise survey, do you really want to ask them the same few questions that are appropriate for everyone else? Or is that a missed opportunity?
Instead of the census, we could ask small communities specific and relevant questions. In remote aboriginal communities, it might not make sense to ask people about their journey to work, but it might make more sense to ask for example about what they eat, which the census does not do.
The census costs a lot – reportedly $440 million last time – with the price tag going up for the 2016 iteration. The entirety of the ABS Budget is just $400 million, with which they put out new data and analysis just about every day.
I do not – DO NOT – support cutting the budget of the ABS to do their important day to day work. But I can see why running a census every five years might seem like a waste of resources that would be better used supporting that work.
The best reason to axe the census would be that it adds little empirical exactitude when obtaining estimates of the great homogenous mass of us, and is too blunt to ask the questions that matter of the smaller communities it covers.
Anyone who has read this far may be interested in my review of the US census, which I was lucky enough to participate in during 2010: The US census incensed us. I sense a lack of consensus.