When propensity to respond to a survey is correlated with key survey variables, nonresponse bias can occur. One method of assessing nonresponse bias is to compare respondents with nonrespondents using auxiliary variables from the drawn sample. A limitation of this method is that many frames have only basic demographic variables, which may be poorly correlated with
response propensity. However, for low incidence and hard-to-reach populations, recontact studies are a popular option, often utilizing rich sampling frames containing behavioral and attitudinal variables from previous surveys. This paper assesses the impact of unit nonresponse in a recontact study of young adults who had recently completed a similar 'seed' study. Both studies were sponsored by the U.S. Department of Defense; the initial study examined attitudes and behaviors pertaining to military recruiting, and the recontact study assessed the awareness of and attitudes toward the Military's advertising campaigns. The seed study consisted of three iterations of a national mail survey of young adults ages 16 to 24, sampled from an address list database which covered more than 90% of the target population. Respondents to the seed study who provided an email address were used as a sampling frame for the recontact study, which was completed online. Using auxiliary variables from the original frame and from responses to the seed study, we examine unit nonresponse in the recontact study to assess differences between respondents and nonrespondents and the impact on key survey estimates. First, we compare characteristics of respondents and nonrespondents on a variety of demographic, attitudinal, and behavioral measures. Where characteristics differ significantly between the two groups, we conduct regression analysis to determine whether these characteristics also significantly predict responses to survey questions in the recontact study. After examining the impact of unit nonresponse, we discuss implications for future research.