Roland-Morris Disability Questionnaire (RMDQ) and average back pain intensity in past week by age, race, sex and education stratified by site. a, c, e, and g display the baseline measure of back-related physical disability (the Roland-Morris disability questionnaire) and b, d, f, h display the baseline measure of pain (a numerical rating scale of average back pain in past week), stratified by site and compared with key demographic variables. a and b depict baseline measures by age; c and d depict baseline measures by race; e and f depict baseline measures by sex; and g and h depict baseline measures by education. Each outcome by demographic boxplot displays the within-group mean (horizontal in the center of the box) +/- the within-group standard deviation (upper and lower edges of the box). The vertical lines display the within-group range of scores.