Assessing research outcomes by postal questionnaire with telephone follow-up. TOTAL Study Group. Trial of Occupational Therapy and Leisure.

Face-to-face assessment of research outcomes is expensive and may introduce bias. Postal questionnaires offer a cheaper alternative which avoids observer bias, but non-response and incomplete response reduce the effective sample size and may be equally serious sources of bias. This study examines the extent and potential effects of missing data in the postal collection of outcomes for a large rehabilitation trial.