Study protocol, sample characteristics, and loss to follow-up: The OPPERA prospective cohort study

When studying incidence of pain conditions such as temporomandibular disorder (TMD), repeated monitoring is needed in prospective cohort studies. However, monitoring methods usually have limitations and, over a period of years, some loss to follow-up is inevitable. TheOPPERAprospective cohort study of first-onset TMD screened for symptoms using quarterly questionnaires and examined symptomatic participants to definitively ascertain TMD incidence. During the median 2.8-year observation period, 16%of the 3,263 enrollees completed no follow-up questionnaires, others provided incomplete follow-up, and examinationswere not conducted for one third of symptomatic episodes. Although screeningmethods and examinationswere found to have excellent reliability and validity, theywere not perfect. Loss to follow-up varied according to some putative TMD risk factors, although multiple imputation to correct the problemsuggested that biaswas minimal. A secondmethod of multiple imputation that evaluated bias associated with omitted and dubious examinations revealed a slight underestimate of incidence and somesmall biases in hazard ratios used toquantify effects of risk factors. Although''bottom line'' statistical conclusions were not affected, multiply-imputed estimates should be considered when evaluating the large number of risk factors under investigation in the OPPERA study. Perspective: These findings support the validity of the OPPERA prospective cohort study for the purpose of investigating the etiology of first-onset TMD, providing the foundation for other papers investigating risk factors hypothesized in the OPPERA project.