Who else responds to postal questionnaires? Are those involved in the subject of the study more likely to do so?

In a postal screen to identify people who had attended hospital outpatient departments, it was predicted that attenders would be more likely to respond than non-attenders. An experiment was set up to explore this. The response rates were 76 per cent among attenders compared with 70 per cent for the others--a difference which did not reach significance at the 5 per cent level.