The effects of failing to include hard-to-reach respondents in longitudinal surveys

Objectives. We sought to determine whether failure to locate hard-to-reach respondents in longitudinal studies causes biased and inaccurate study results. Methods. We performed a nonresponse simulation in a survey of 498 low-income women who received cash aid in a California county. Our simulation was based on a previously published analysis that found that women without children who applied for General Assistance experienced more violence than did women with children who applied for Temporary Assistance to Needy Families. We compared hard-to-reach respondents whom we reinterviewed only after extended follow-up effort 12 months after baseline with other respondents. We then removed these hard-to-reach respondents from our analysis. Results. Other than having a greater prevalence of substance dependence (14% vs 6%), there were no significant differences between hard- and easy-to-reach respondents. However, excluding the hard to reach would have decreased response rates from 89% to 71% and nullified the findings, a result that did not stem primarily from reduced statistical power. Conclusions. The effects of failure to retain hard-to-reach respondents are not predicable based on respondent characteristics. Retention of these respondents should be a priority in public health research.