The effects of aggregation on the reliability of measures of academic performance were explored in two studies. In the first study, 30 elementary-age children were tested four times on the same forms of the Woodcock Reading Mastery Tests and the Ginn 720 Reading Passage measures. Group stability coefficients, within-subject reliability coefficients, and group correlations between variables each were calculated on the basis of one or two testings and then on the basis of aggregations over four testings. On the standardized measure and on the oral passage reading correct rate score, aggregation had little impact; however, on the oral passage reading error rate score, aggregation substantially increased all reliability indices. In the second study, 78 children were tested 10 times on alternate forms of two reading measures and one written expression measure. Group stability coefficients were calculated on the basis of 2, 4, 6, 8, and 10 testings. For the oral words-in-isolation reading correct score, aggregation had little effect, whereas aggregating over occasions and test forms dramatically improved the stability of the oral words-in-isolation reading error score and the written expression score. The reliability and criterion validity of short, simple measures demonstrated their suitability as measures of academic performance. (Author/PN)