1) increased sample size, multiple institutions, multiple age ranges, multiple programming languages, multiple native languages, correlation with Spearman's g
2) controlling for confounders (see "Table 1" of all medical trials)
3) randomization of instruction
4) better history of pre-exposure (surely someone in a university computer class is taking it out of an interest that developed before enrollment)
5) parental professions, levels of education, and annual earnings
6) latency effects -- did people "come around" later, how long do the effects last?
7) frequency effects -- does a burst of lots of programming exposure kick someone over a knee in the curve (as is done in language schools)
8) amplitude effects -- does intense instruction yield intense results?
Overall I liked it.