"The program included home visitations in addition to
preschool, making it impossible to determine
whether the preschooling alone"

"The intervention
continued through third grade
for some students, and involved tutoring,
speech therapy, and medical services that
are not a part of current preschool proposals
and would dramatically raise the
costs and difficulties of expanding such a
program statewide."

"The research design simply
matched children whose parents chose
to participate in the CPC program with
those who did not.20 It was not a random-
assignment study, which means
that subtle differences between the two
groups—which can go undetected by
survey responses, income data, and other
rough measures—might explain differences
between the two populations.21
There is no way to determine if the program
had an impact or if the families
who participated were simply different
from those who did not participate."

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