Number of deaths or number of deaths per capita

Pablo Haya writes:

Currently, there is a lot of data analysis in the news media showing multiple aspects of the COVID-19 crisis. Many of them compare the virus spread and evolution between different countries, or between different regions within each country. They use to compare the absolute frequency of several metrics such as confirmed cases or deaths, or they normalize these metrics by population. In this case, which of these alternatives is more convenient for making a fair comparison?

According to John Burn-Murdoch, from Financial Times, adjusting for population size is a bad idea as COVID-19 is a transmissible disease, and when you normalize, higher per-capita numbers just mean smaller country, not anything different about how that country’s dealing with COVID-19.

What do you think?

My reply: When comparing countries, I think it makes sense to look at per-capita rates, not absolute numbers, but as Murdoch points out in the linked thread, you could also compare regions within a country. The disease does not respect political boundaries (except to the extent that people are not traveling between countries, but then they might not be traveling between regions within countries either). If you’re comparing rates of growth on the log scale, then it doesn’t matter whether or not you divide by population, because that’s essentially constant over time. Murdoch is right that if you make a graph, starting the time axis at the first death within a country, then per-capita comparisons can be misleading. But if you start the time axis at a constant per-capita rate, the denominators will again cancel out. I guess the general point is that there’s no absolute right or wrong way to make these plots: different graphs show you different aspects of the data.