Attrition in an exercise intervention: a comparison of early and later dropouts

OBJECTIVE: To identify reasons for dropout and factors that may predict dropout from an exercise intervention aimed at improving physical function in frail older persons. DESIGN/SETTING: An 18-month randomized controlled intervention in a community setting. The intervention comprised 2 groups: class-based and self-paced exercise. PARTICIPANTS: 155 community-dwelling older persons, mean age 77.4, with mildly to moderately compromised mobility. MEASUREMENTS: The primary outcome measure was dropout. Dropouts were grouped as: D0, dropout between baseline and 3-month assessment, and D3, dropout after 3-month assessment. MEASUREMENTS: Measurements of demographics, health, and physical performance included self-rated health, SF-36, disease burden, adverse events, PPT-8, MacArthur battery, 6-minute walk, and gait velocity. RESULTS: There were 56 dropouts (36%), 31 in first 3 months. Compared with retained subjects (R), the D0 group had greater disease burden (P = .011), worse self-perceived physical health (P = .014), slower usual gait speed (P = .001), and walked a shorter distance over 6 minutes (P<.001). No differences were found between R and D3. Multinomial logistic regression showed 6-minute walk (P<.001) and usual gait velocity (P<.001) were the strongest independent predictors of dropout. Controlling for all other variables, adverse events after randomization and 6-minute walk distance were the strongest independent predictors of dropout, and self-paced exercise assignment increased the risk of dropout. CONCLUSIONS: We observed baseline differences between early dropouts and retained subjects in disease burden, physical function, and endurance, suggesting that these factors at baseline may predict dropout. Improved understanding of factors that lead to and predict dropout could allow researchers to identify subjects at risk of dropout before randomization. Assigning targeted retention techniques in accordance with these factors could result in decreased attrition in future studies. Therefore, the results of selective attrition of frailer subjects, such as decreased heterogeneity, restricted generalizability of study findings, and limited understanding of exercise effects in this population, would be avoided.