“A lie that is half-truth is the darkest of all lies.”
― Alfred Tennyson
Last week, a study published in the Journal of the American Medical Association looked at the prevalence of childhood obesity over the last 10 years. The study, performed by Cynthia Ogden and colleagues at the CDC, aimed to describe the prevalence of obesity in the US and look at changes between 2003 and 2012. The study itself had several interesting findings, not least among them that the prevalence of obesity seems to have stabilized in many segments of the US population. However, they made one observation that caught the media’s attention:
“There was a significant decrease in obesity among 2- to 5-year-old children (from 13.9% to 8.4%; P = .03)”
This is where things get interesting, as the focus was not on the 5.5 percentage points difference. Instead of reporting the absolute difference, i.e. how much something changed, news outlets focused on the relative difference, i.e. how much they changed compared to each other. In that case, it would be (5.5/13.9 =) 40%. Which is much more impressive than the 5.5% change reported in the study. So you can guess what the headlines loudly proclaimed.
Mr Epidemiology 