Research by Cook, Eignor, and Taft (1988) has clearly demonstrated the need for equating certain achievement tests using samples of students who take the new and old forms at comparable points in the school year. In their study, Cook et al. equated two forms of an achievement test in biology using two different sample combinations, a spring new-form/fall old-form combination and a fall new-form/fall old-form pairing. Equating results were quite different across the two sample combinations and all equatings, based on a variety of methods, and those done using the spring new-form/fall old-form combination provided unacceptable results. However, results from recently completed research by Lawrence and Dorans (1988) suggest that matching samples of differing ability (such as the spring new-form and fall old-form samples in the Cook et al. study) on an available covariate prior to equating may improve equating results.