 I find that providing access to Head Start can have positive effects on test scores and that these effects can negatively depend on whether access to an alternative preschool was available.
<meaning-changed>  I find that providing access to Head Start can have positive effects on test scores and that these effects can negatively depend on whether access to an alternative preschool was available.
 I find that such a policy affects a large number of children who take up the offer, and that they subsequently have positive effects on test scores and that these effects can negatively depend on whether access to an alternative preschool was available.