Kids are really fat, we think
My statistical knowledge, and my familiarity with public-health practices, are both at the level of educated layperson; in other words, I’m not an expert. That makes the flaw in this Los Angeles Times piece by Karen Kaplan all the more concerning, since even I can spot it.
The Centers for Disease Control and Prevention define childhood obesity not according to body mass index (as it does for adults) but according to how a child’s BMI compares with that of other kids of the same age and gender. Children who have a BMI at or above the 95th percentile are considered obese, and those with a BMI between the 85th and 95th percentiles are considered overweight. In 2012, fully 18% of children between the ages of 6 and 11 were considered obese, up from 7% in 1980, the study authors wrote. [Formatting in original.]
The 95th percentile, by definition, is the point below which 95% of the population lies; anyone “at or above the 95th percentile” is in the top 5% of the population, by definition. If obesity in children means (per the CDC) being in the top 5% of the population for BMI, then 18% of the population cannot be obese. How much of the population can be in the top 5% of the population for BMI? Exactly 5%.
The CDC’s definition of childhood obesity, which the Times is faithfully reproducing from the CDC website, is itself unhelpful. Unless the 95th percentile is defined historically, childhood obesity figures from the CDC cannot even tell us whether children are becoming more or less healthy. Exactly 5% of children will always be obese, every single year, every single cohort. And being obese for a child (per the CDC) has no clear or established relationship to health problems. Being in the top 5% for BMI simply means that a child is heavier than her peers. If all her peers are below a healthy weight, then perhaps she is one of the few who is healthy — despite being “obese”.
A public-health expert might be able to look at the CDC’s website and explain how the CDC is oversimplifying for a lay audience. But apparently the CDC is relying on ignorance (and public awe) of statistics to mask that oversimplification, and the LA Times is willing to print articles that put that ignorance and awe on display.
— O.T. Ford
Source: Los Angeles Times