Anyone who has taken his child to a pediatrician will have seen a growth chart. Here is one for boys aged 2 to 5 (pdf). It shows, for example, that the median height of a three year old boy is about 96 cm, and the 15th and 85th percentiles are 92 cm and 100 cm, respectively. In other words, the chart recognizes that there is a distribution of heights. The height of most (70%) three year old boys will be between 92 cm and 100 cm. The chart also gives the 3rd and 97th percentiles, which indicate that the height of the vast majority (94%) of three year old boys will be between 89 cm and 103 cm.
Suppose it were up to you to define a normal height range for three year old boys — something that parents could use to determine whether their child was of normal height for his age. What range would you use?
I think I would pick the 15rd and 85th myself. Would anyone choose the 50th and 97th percentiles? Would you be comfortable then advising parents that any child whose height was below the 50th percentile was “short or stunted”?
In effect, this is what the government has done with weight for adults. The NHANES I study measured the height and weight of 23,808 Americans (16,165 adults and 7,643 children) from 1971 to 1975 — before the onset of the obesity epidemic. The “healthy BMI range” of 18.5 to 25 corresponds to the 4th and 53rd percentiles of the adult population. So by today’s standards, 47% of the population in the early 1970s was “overweight or obese”!
If I were to define a normal BMI range, I would choose perhaps the 25th and 75th percentile of the BMI distribution of a reference population. If we use the NHANES I data as the reference population, those percentiles are 22 and 28.
Filed under: obesity |