Several memes have been making the social media rounds based on a New York Times headline: “Obesity Dropped 43% Among Young Children In Decade, Study Finds” When you look up the study that’s referenced, the conclusion states “Overall, there have been no significant changes in obesity prevalence in youth or adults between 2003-2004 and 2011-2012″
So just what in the Sam Hill happened here? It’s something that my favorite reporting methods professor in college used to call “fun with statistics” and it most often occurs when someone (a researcher, reporter, company, or *cough* First Lady *cough*) wants a particular outcome and so they just finesse the numbers to get it.
In this case, what the researchers found was that (to quote the NYT article) “About 8 percent of 2- to 5-yearolds were obese in 2012, down from 14 percent in 2004.” (In actuality the study states right there in the abstract that it was 13.9% to 8.4% which is a delta of 5.5% rather than 6% but why would accuracy be important here?) So how did a 6% (actually 5.5% but who’s counting) difference become a 43% change in the New York Times headline? Through the magic of absolute vs. relative change!
Absolute change is the difference in the item being studied at two different times:
[% of fat 2-5 year olds in 2012] – [% of fat 2-5 year olds in 2004] = Absolute change
.08 – .14 = -.06
Relative change is the absolute change calculated as a percentage of the indicator in the first time period.
([% of fat 2-5 year olds in 2012] – [% of fat 2-5 year olds in 2004]) / [% of fat 2-5 year olds in 2004] * 100%
(.08 – .14) / .14 * 100 = -42.85% (they rounded to -43%)
Often when someone is trying to make something seem like a bigger deal than it is, they will use relative change rather than absolute change. And this doesn’t even take into account the margin of error, number and effect of dropouts etc. That’s all the formulas in this post, I promise (though if you want to see further discussion of the fuzzy math, there’s a good piece in Forbes, though trigger warning for obesity epi-panic language). Something that is worth noting is that for all of this talk about OMGDEATHFAT increasing so much, this study actually found no significant change since 2004, but that’s a subject for another blog.
This is certainly not the first time that this has happened. Allergan has used “Fun with Statistics” to try to sell lapbands. We’ve even seen this kind of reporting be not only crappy, but actually dangerous. So why am I talking about this now?
First of all, because it’s making the rounds again and every time I see it, it pisses me off. Also since focusing on the weight of children has been a pet project of Michelle Obama, who’s time at the White House is winding down, I expect that we may see a lot more attempts to suggest that focusing on the weight of children – and telling fat kids that we’re all trying to create a world where nobody who looks like them exists – has been a fantastic idea, despite the evidence.
This kind of reporting drives the discussion and policy – including policies that body shame children as a substitute for actually supporting their health – so it’s important that we know that it’s always reader beware, that we can and should ask questions, and that we can and should hold the people doing this reporting accountable.
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