In addition, confounding may also arise from factors other than polysubstance abuse, including birth factors such as prematurity and head circumference, and adverse environmental factors such as poverty and poor parenting. Some studies control for confounding variables by study design, such as only selecting subjects exposed to the drug of interest which is feasible with, for example alcohol160 or cigarette use194, or matching subjects for factors such as gender140. Factors such as poverty are more problematic for studies of cocaine and methamphetamine, whereas alcohol, tobacco and marijuana can be studied in impoverished and non-impoverished populations. In addition, since most of these studies are cross sectional rather than longitudinal, information on the past history of the children may be sparse or based on recall and subject bias. Documentation of drug use during pregnancy is typically based on self report without verification by toxicological analysis, and subjects may not reveal their true drug use, especially in the case of illegal drugs. Thus, there can be undetected drug users in the comparison group and information on use of other drugs or quantity and
