50% of kids are on food stamps at some point in their childhood

Mark Rank and Thomas Hirschl recently published an estimate that 50% of American kids are on food stamps at some point during their first twenty years of life. Their estimate is based on an analysis of data from the Panel Study of Income Dynamics, from 1968 through 1997.

This news article by Lindsey Tanner provides a good overview.

The survey followed up families annually, thus there are kids in the study who were included at age 1, 2, . . ., 20. From this you can easily just count the proportion who were never on food stamps, the proportion who were on food stamps for one year during the first 20 years of their lives, the proportion who were on food stamps for exactly two years, etc.

Rank and Hirschl don't quite do this; instead they use all their data to estimate the probability of being on food stamps at age 1; then they use all the kids who were in the study for ages 1-2 to estimate the prob of being on food stamps at age 2, if they were not on food stamps at age 1; . . . and for their last step, they use the subset of kids who were in the study continuously for ages 1-20 to estimate the prob of being on food stamps at age 20, for kids who were not on food stamps for the first 19 years. Put these together and you can figure out the probability of ever having food stamps.

This is all fine--it's an efficient use of the data they have--but I'd feel a bit more confidence in Rank and Hirschl's estimates if they would cross-check by doing some raw-data calculations based on the subset of kids who were in the study continuously for ages 1-20. That's a crucial component in any applied statistical analysis--the continuous thread connecting the raw numbers to the final estimate--and I always like to see it, especially for a politically-charged subject such as this one. But really this isn't much different from my comment on the basketball halftime study: I'll believe the fancy analysis a lot more if I see the connection to the data.

Here are the key results from the study:

(from Table 1): 12% of newborns were on food stamps. 49% of kids were on food stamps for at least one year between ages 1-20. 23% of kids were on food stamps for at least 5 years.

(from Table 2); 8% of white newborns and 33% of black newborns were on food stamps. 37% of white kids and 90% of black kids were on food stamps for at least one year between ages 1-20.

(from Table 3): Among the black kids of unmarried parents where the head of household did not graduate from high school, 99.6% were on food stamps.

Again, I don't know how much to believe these numbers, but I assume that they're not too far from what was really happening in those years.

Also, whassup with those superfluous decimal places? "22.8%" and all the rest? Doesn't anybody teach these people about sampling variation and significant digits? (I guess I should let them off the hook, given that the entire economics profession seems to have this problem too.)