Most of the studies examined for this review include a measure of prior achievement as a predictor of later achievement. Although this variable was typically entered into the analysis to "control" for selection bias, all of the studies that included a measure of prior achievement found it to be a statistically significant predictor of later academic success. For instance, Jordan and Nettles (1999), in an analysis of data from the National Educational Longitudinal Study of 1988 (NELS), found that 10th grade reading achievement predicted math and science achievement in the 12th grade. This relationship was also documented by McNeal (1999) and Gamoran (1992), using data from High School and Beyond. Further, this relationship consistently held while controlling for other important background factors, such as family income, parental education, and race. One study found that earlier grades predicted later grades (Gutman and Eccles, 1999), while another found that reading and math test scores were the strongest predictors of later achievement scores (Gortmaker, Salter, Walker, and Dietz, 1990).