    Sentence 1: Usually, sites For progran effects case  studies should be selected with great care For  criteria such as whether there is evidence that the  progran has been inplenented at the site, uhether  the site has been subjected to changes that could  have the sane effects as the program or that could  nask its effects, and hou the addition of this site to  the group of sites being studied supports the  generalizability of the Findings.    Sentence 2: Sone sites have undergone changes  since the Last program implementation on those  sites.