""However, there
are a number of problems with the Perry Preschool
Program and the associated analyses
that render them unreliable and unsuitable
for estimating the effects of the large-scale,
conventional preschool programs currently
under consideration in state legislatures:

Yale psychologist
Edward Zigler, among others,
notes that “the Perry Project poses a number
of methodological difficulties” that
call into question any conclusions about
the effects of the program, positive or negative.
Assignment to the preschool treatment
and control groups was not completely
random, an absolute requirement 
for an ostensibly randomized experiment.
And the children in the preschool program
had to have parents home during
the day—a requirement “resulting in a significant
difference between control and
intervention groups on the variable of
maternal employment” that also calls the
project’s results into question

https://object.cato.org/sites/cato.org/files/pubs/pdf/pa641.pdf