Responses to anonymous questionnaires concerning sexual behavior: a method to examine potential biases.

OBJECTIVES: Low response rates to voluntary surveys raise questions about how representative the responses are. We compared the behavior and attitudes of responders, willing and reluctant, and nonresponders to anonymous questionnaires about behaviors that might expose participants to the human immunodeficiency virus (HIV).
METHODS: Questionnaires were sent to 1080 Danish adults 18 through 59 years including explicit questions about sexual acts and illegal drug use. Identical questionnaires were sent to 3600 other Danes, similarly chosen; packets sent to these persons included cards to be returned separately informing us that they had responded. Questionnaires were sent twice more to nonresponders, who, if they then responded, were considered reluctant responders. One hundred nonresponders were telephoned and asked why they had refused to respond.
RESULTS: Enclosing return cards did not affect initial response rate, but prompting boosted replies from 52% to 73%. However, behaviors were generally similar among initial and reluctant responders. One third of nonresponders agreed to respond if we wished (total potential response: 82%). In general, the reasons for nonresponse did not suggest that the life-styles of nonresponders placed them at risk for HIV infection.
CONCLUSIONS: This method provides a simple, inexpensive approach to improving response rates and learning about the biases of reluctant responders and nonresponders.