Assessing research outcomes by postal questionnaire with telephone follow-up

Background 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. Methods Questionnaires containing a number of established scales were posted to participants in a trial of occupational therapy after stroke. Response was maximized by telephone and postal reminders, and incomplete questionnaires were followed up by telephone. Scale scores obtained by imputing values to questionnaire items missing on return were compared with those achieved by telephone follow-up. Findings Response to the initial posting was 60%, rising to 85% after reminders. Participants receiving the experimental treatment were more likely to respond without a reminder. There were no significant differences on any known factors between eventual responders and non-responders. Of the questionnaires, 43% were incomplete on return: partial responders were significantly different to complete responders on baseline disability and home circumstances. Of the incomplete questionnaires, 71% were resolved by telephone follow-up. In these, the scale scores achieved by telephone were generally higher than those derived by conventional imputation. Conclusion Postal outcome assessment achieved a good response rate, but considerable effort was needed to minimize non-response and incomplete response, both of which could have been serious sources of bias.