wiliki's linked study had various methodological flaws.  

Many analysts discount the reliability of the Perry study. For example, Hanushek and Lindseth (2009), among others, claim that the sample size of the study is too small to make valid inferences about the program. Herrnstein and Murray (1994) claim that estimated effects of the program are small and that many are not statistically significant. Others express the concern that previous analyses selectively report statistically significant estimates, biasing the inference about the program (Anderson (2008)).

There is a potentially more devastating critique. As happens in many social experiments, the proposed randomization protocol for the Perry study was compromised. This compromise casts doubt on the validity of evaluation methods that do not account for the compromised randomization and calls into question the validity of the simple statistical procedures previously applied to analyze the Perry study.