“…Although some studies have included only subjects with urine screens positive for cocaine metabolites at delivery (Chouteau, Namerow, & Leppert, 1988;Gillogley, Evans, Hansen, Samuels, & Batra, 1990;Hollinshead et al, 1990;McCalla et al, 1991;Neerhof, MacGregor, Retzky, & Sullivan, 1989;Oro & Dixon, 1987;Spence, Williams, DiGregorio, KirbyMcDonnell, & Polansky, 1991), this selection process excludes admitted users and misclassifies nonadmitting users when these women did not use within a few days prior to delivery. For example, Spence et al (1991) included 18 (5.2%) women, who admitted prenatal cocaine use but had negative urine specimens at delivery, in the "cocaine-negative" group for comparison with urine-positive users.…”