A number of statistical “adjustment” strategies have been suggested in an attempt to remove the bias attributable to these confounding factors. These techniques are based on a static model in which the outcome is a relatively simple function of various inputs, one of which is the treatment. In educational studies, one of the principal “inputs” is generally a measure of the outcome variable prior to the program. Statistical adjustments are based primarily on the observed relationship between these pretest scores and the posttest scores measured after the intervention.