Statistics Done Wrong

Chapter 7. Continuity Errors

So far in this book, I’ve focused on comparisons between groups. Is the placebo or the drug more effective? Do intersections that allow right turns on red kill more people than those that don’t? You produce a single statistic for each group—such as an average number of traffic accidents—and see whether these statistics are significantly different between groups.

But what if you can’t separate test subjects into clear groups? A study of the health impacts of obesity might measure the body mass index of each participant, along with blood pressure, blood sugar, resting heart rate, and so on. But there aren’t two clear groups of patients; there’s a spectrum, from underweight to obese. Say you want to spot health trends ...