Usually, sites for program effects case studies should be selected with great care for criteria such as whether there is evidence that the program has been implemented at the site, whether the site has been subjected to changes that could have the same effects as the program or that could mask its effects, and how the addition of this site to the group of sites being studied supports the generalizability of the findings.
The addition of a site to an existing group of sites under study has to support generalizability of the findings.