# Using the right denominator

We go on and on about denominators on StatsChat: the right way to report things that happen to people is usually a rate per capita rather than a total, otherwise you end up saying that Auckland has the highest number of whatever it is in New Zealand. You do have to use the right denominator, though.

The Vatican City has the world’s highest crime rate.

That’s because the permanent population is less than 500, but the daily tourist population is about 100 times larger. The right denominator would be the tourist population.

In most countries this isn’t really an issue. For example, in New Zealand,which has a lot of tourism, short-term visitors are only about 5% of the population. Even in the Cook Islands, residents outnumber tourists.

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 »

I also note that per capita is not only for comparing across space, but also across time. For example, I am always puzzled that growth in total GDP is reported and heavily commented on, not growth in GDP per capita. Anyone should be able to work out that, ceteris paribus, increases in population will increase GDP. The latest migration figures show a annual net increase of 49,800 which on a population of 4.5 million implies that if total GDP didn’t increase by at least 1% annually something isn’t going well.

3 years ago