Because parents of children who left the study before age 13 years consistently endorsed higher rates of tics than those who remained at age 13 (Table S1, available online), we examined the factors related to attrition in the sample (Table S5, available online). Female gender, nonwhite ethnicity, lower maternal age, and markers of lower socio-economic status such as housing tenure and maternal education were all associated with loss to follow-up before age 13.
