When a program evaluation is conducted on the basis of gains made in standardized achievement test scores between a pretest and a posttest, serious bias can result where a floor effect exists on one or both of the test levels. Based upon the expanded standard scores on the Comprehensive Tests of Basic Skills from three successive regular spring test administrations and a special mid-period out-of-level test administration, this study examined score distributions for schools in a large city school system to determine if their compensatory program evaluations were biased by a floor effect. The distributions were examined for changes in location, variation, and skewness as students moved across time, taking different levels of the test at different times. The pattern of distributional characteristics for both reading and mathematics over the four testing periods indicated that a floor effect was quite unlikely to bias group comparisons in the upper elementary grades under study (grades 5 and 6). (Author/CP)