I have been away (again) and out of internet contact most of the day, dealing with an unhappy family event. So this post is short but illustrates an important point that comes up frequently in epidemiology: the difference between risks and absolute numbers. The illustration is not medical, but I think sharper because of it:
The right-wing blog Gateway Pundit says Christians are the target of more hate crimes in the U.S. than Muslims:
In the real world… Hate crimes against Muslims have steadily declined since 2001. Today there are more reported hate crimes against Christians in the United States than Muslims.
That’s a reference to the fact that there were 115 reported hate crime incidents against Muslims in 2007, according to the FBI, and 118 against Christians.
Now, this would be a good time to keep in mind that 78 percent of American adults are Christian, and 0.6 percent are Muslim. In other words, there are about 130 times as many Christians in the US as there are Muslims. And yet there were essentially the same number of reported hate crimes against Muslims as against Christians in 2007.(Jamison Foser, Media Matters)
What this means is that the risk of being a victim of a hate crime is 130 times higher if you are a Muslim compared to a Christian. Of course we could avoid this problem (along with anti-Semitism) by just forgetting about religious affiliation in the first place. God wouldn’t care, any more than the Tooth Fairy would care if you stopped believing in Her (or is the Tooth Fairy a Him or an It?).
Back to the topic. Which is the most appropriate measure, absolute numbers or risks? It depends on what you want to show. The numbers of deaths in a city may not reflect the risk of dying if you live there for many reasons, the most obvious being that cities have different sized populations. But the absolute number of deaths may be extremely relevant for determining how many undertakers you need.
There isn’t one right answer, but there are certainly wrong ways to use absolute numbers and risks. It’s a topic that deserves a deeper discussion. But not now. I’m out of steam. Tomorrow’s another day. A better one, I hope.