The New York State Experimental Prekindergarten Program (PreK) evaluation studied successive waves of children while they were in PreK, and as they progressed through the third grade. For each wave, two control groups were obtained: (1) A waiting-list control group, children in districts offering PreK programs who were identified as eligible for the PreK program. (2) An other-district control group, children in selected districts not operating PreK programs. To attempt to attain comparability between PreK children and children in the other-district control groups, selection took place at two levels: control districts were selected so as to resemble PreK districts as a group, and within those districts, children were selected according to the criteria used in admitting children to the PreK program. Restrictions in the availability of comparable districts and reluctance on the part of a large number of districts to participate made the match of control group and PreK group inexact. Differences among the control districts in the attrition of children available for the study may further reduce the comparability of the control group and PreK group. (Author/MV)