Transitioning from a ten-year legacy of RDD sampling, the probability-based online KnowledgePanel® uses an address-based sample (ABS) frame for mail-based recruitment of panel members. This paper analyzes nonresponse through a statistical analysis of “late responders” at two stages of the panel survey process: panel recruitment and the online surveys themselves. The characteristics of “late responders” provide a statistical, however imperfect, window into the characteristics of non-responders at these two stages. Furthermore, any differences between early and late responders would have implications for nonresponse bias in the event the field period is shortened for either the panel recruitment or the online surveys. First, to examine late responders to the ABS-sourced panel recruitment, we compared full demographic and other profile-based information that had been collected among both early and late responders to the mail-based recruitment. After these mail-based recruitment efforts, non-responders are contacted using a telephone-based recruitment whenever a landline phone number can be matched to a residential address. Second, to examine late responders to KnowledgePanel web surveys themselves, we analyzed the samples for three large-sample online surveys. Preliminary research found that late responders to the mail recruitment and panel surveys are disproportionately non-white (p-value<0.05), live in three- or more-person households (p-value<0.1), and younger adults (p-value<0.01). The findings will be useful in designing more effective recruitment and online methods encourage response (such as using a targeted segmentation of messages and incentives) in order to minimize response bias.