Text stimulus (table): Table titled 'Participation From Assignment to Follow-Up.' Columns: Condition, Randomly Assigned, Completed Follow-Up, Dropped Out, Dropout Rate. Treatment row: 200, 130, 70, 35%. Control row: 200, 180, 20, 10%. Note: 'Reported outcome analysis included follow-up completers only.' What is the most serious threat to the reported completer-only comparison?