Greg Cochran writes at WestHunter:

The New York Times just put out a story titled “Overdose Deaths Exceeded Half a Million in 2014”. As so very often on reading something in the Times, I said to myself “that’s wrong.” Because it had to be wrong – off the top of my head I knew that less than three million died from any reason each year in the US: pop of ~300 million, three score and ten, population growth, QED. No way that big a fraction died from overdoses, and of course it was wrong: the real number is something like 50k, mostly prescription drugs. Nobody at the Times noticed it at first. I don’t know that they ever did notice it by themselves- likely some reader brought it to their attention. But this happens all the time, because very few people have a picture of the world in their head that includes any numbers. Mostly they don’t even have a rough idea of relative size.

I find it useful to store in my head a number of highly stylized but reasonably accurate interconnected numbers.

For example: 4 million is a useful round number estimate of the number of Americans of any particular age: newborns, 4th-graders, 21-year-olds, etc. If the average person lives to be 80, you can multiply 4 million by 80 and get 320 million, which isn’t too far off the total population. That simple little web of numbers makes it hard to make order of magnitude mistakes, as this NYT article fell prey to. (If I forget either 4 million or 320 million, I can get to the forgotten number using the remembered number and a life expectancy of 80 years.)

What other numbers do you find useful to memorize to give yourself a still point in the turning world?