Unfortunately, this study doesn't take into account other factors that might have affected the findings. For example, since it only measures public school children, it is possible the decline is entirely due to a change in the make-up of the public school population. Has the prolonged recession resulted in more children shifting from private schools to public schools? These likely more affluent students typically have a lower rate of obesity.

Unless you measure the entire population of school children, you aren't getting a true picture. In fact, the rise in obesity in counties where the lowest percentage of students are enrolled in private schools is consistent with the possibility that there has not been a drop in obesity, but rather a shift in what is being measured.