Dropout in a longitudinal, cohort study of urologic disease in community men

During 1989–1991, 2,115 randomly selected Caucasian men, ages 40–79 years from Olmsted County, Minnesota were enrolled and followed with questionnaires biennially; 332 men were added in follow-up. A random subset (~25%) received a urologic examination. Baseline characteristics including age, benign prostatic hyperplasia (BPH) symptoms, comorbidities, and socioeconomic factors were compared between subjects who did and did not participate after the twelfth year of follow-up.Of the 2,447 men, 195 died and were excluded; 682 did not participate in 2002. Compared with men in the 40–49 year age group, men ≥ 70 years of age at baseline had a greater relative odds of dropout, 2.65 (95% CI: 1.93, 3.63). In age-adjusted analyses, relative to men without stroke, men who had suffered a stroke had a higher odds of dropout, age-adjusted OR 3.07 (95% CI: 1.49, 6.33). Presence of at least one BPH symptom was not associated with dropout, (age-adjusted OR 1.12 (95% CI: 0.93, 1.36)).These results provide assurance that dropout was not related to primary study outcomes. However, factors associated with dropout should be taken into account in analyses where they may be potential confounders.Attrition becomes a major concern in long-term studies if those who drop out differ systematically from those who continue to participate. Common statistical methods used for cohort studies assume there is no difference between subjects who drop out of the study and those who continue to participate. However, if this assumption is false, the validity of the study may be jeopardized and the difference between the observed values and "true" values may increase with study follow-up [1]. This is of particular concern if disease severity influences study participation. Losses in long-term studies may be due to refusals to continue in a study, inability to trace individuals for follow-up, and deaths.Few studies of urologic conditions, such as benign prostatic hyperplasia (BPH), have examined dropout of st