How averages get usurped by policymakers

This WSJ reporter doesn't much like statisticians, growth charts, and CDC (link). Her headline is "Is Baby Too Small? Growth Charts Make It Hard to Tell." This is her lede:

Elias Thorsteinsson weighed 6 pounds, 11 ounces, when he was born in January 2010. That put him in the 25th percentile of U.S. babies, meaning 25% of babies were smaller than he was at birth. But he didn't gain weight as quickly as other infants and dropped to the 1st percentile on the pediatric growth chart when he was 6 months old...Elias gained weight rapidly once he started eating solid foods. His head grew even faster—to the 99th percentile on the head circumference chart. His pediatrician wanted to do an MRI to rule out brain abnormalities. But Elias was developing normally, so Mrs. Stebbins refused. "We'd been through the wringer with these percentiles," she says. "I said, 'I have a big head. His father has a big head. He'll grow into it.' "

Is this damned lies and statistics or is this calling everything that has four legs a pig?

She goes on to complain, among other things, the existence of CDC and WHO standards which are not identical, and the existence of pediatricians who don't "always take the time to fully explain what the growth charts mean". (my italics)

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The history of growth charts (height and weight tables) is fascinating, and not widely known.

They were pioneered by actuaries rather than doctors. In other words, they were first created for commercial reasons -- to enhance corporate profits of insurers -- rather than for health reasons.

The first charts were descriptive. It merely gave the body shape for the "average" person of a certain age and gender. Eventually, these charts became prescriptive - people who are not average are rated as "overweight", "obese", "underweight", etc. It's a big difference between knowing you're 50% above the average and being told you have to slim down, which is to say, you should be average.

Now, it appears that the growth charts may become detached from reality. The reporter includes an unattributed quote from the CDC: "The CDC says it doesn't plan to adjust its charts because it doesn't want the ever-more-obese population to become the new norm." This sounds like the standards will now be detached from the data, and simply be whatever the CDC wants us to weigh.

 

PS. This is not an anti-CDC post. There is an interesting problem here relating to using indices. If we keep using average BMI as the "target", and we are failing to revert to the mean, then the "target" will keep moving.

 

Comments

[Gary Hewitt]

They've finally posted the underlying data for the WHO charts. (I say finally; I last looked when my children were closer to 2 years old, a decade ago). http://www.cdc.gov/growthcharts/who_charts.htm

This site also has some interesting commentary on the different bases behind the WHO charts (which are explicitly intended to be normative) and the CDC charts (which are descriptive of some white, bottle-fed Ohio children as I recall). Full desription here http://www.cdc.gov/mmwr/preview/mmwrhtml/rr5909a1.htm

In any event. I always thought it was difficult to use these charts becuase of the enormous amount of error inherent in the measurements for an individual child. Look at the difference between 25th and 50th, or 50th and 75th percentile for under-1-year-old children - it ranges from about .5 to .65 kg or so. That's roughly the difference between a child with a full stomach and empty diaper (heavy) and a child with a full diaper (or thrown away diaper) and empty stomach. Meaning a child can move 20 or 30 percentile points just by doing its daily business.

[zbicyclist]

This is a particular problem given the obsessiveness of first time parents with where their child is. [pleading guilty myself]