Objective. To explore the response rate benefits and data limitations of mixing telephone and web survey modes in a health-related research study.

Data Sources/Study Setting. We conducted a survey of salaried employees from one large employer in the manufacturing sector in the summer of 2005.

Study Design. We randomized 751 subjects, all of whom had previously completed a web survey, to complete a health-related survey either by telephone (with web follow-up for nonrespondents) or over the web (with telephone follow-up).

Principal Findings. Survey response rates were significantly higher for the mixed mode survey than they would have been if we fielded either an exclusively telephone or web survey (25 and 12 percent higher, respectively). Telephone respondents were more likely to provide socially desirable responses to personal lifestyle questions and web respondents provided more missing data when questions were difficult or did not relate to their circumstances. Telephone respondents were also more likely to "agree" to knowledge statements and provide the same response across a series of items with similar response scales than were web respondents.

Conclusions. Mixed mode telephone/web surveys can substantially boost response rates over single-mode surveys. Modal differences in responses can be minimized by handling missing data options consistently in the two modes, avoiding agree/disagree formats, and not including questions on personal lifestyle or other sensitive issues.