Last week, a journalist asked me a question in connection with the recent flurry of stories on changes in childhood obesity percentages in the period from 2001 to 2012. When I looked into it, what struck me was that a category defined as "BMI at or above the 95th percentile" applied to about 15-17% of the population throughout the period discussed.
This sounds like a statistical approximation to Garrison Keillor's joke about his home town, where "all of the children are above average". But the normative percentiles are based on data from an earier time, and so it's perfectly logical that 17.1% of the age-2-19 sample in the 2003-2004 period should be at or above the 95th percentile for the 1963-1994 period. This is just a symptom, after all, of the famous "obesity epidemic".
Still, I remained curious about just when this large change really took place. (Most of) the raw data is available on line from the CDC, and I decided to spend an hour or so satisfying my curiosity about what is going on here: has there actually been a gradual climb over 50 years, which looks steep when a threshold derived from 1963-1994 is used for data from 2003 to the present? Or was there a steeper climb over a narrower stretch of time?
I found a clear answer to this question. But when I looked into it further, I found some additional information that made me wonder whether there has really been any change over time at all.
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