Relevance & Research Question: Online survey researchers are often confronted with the question for how long they should set the field period. On the one hand, a longer fielding time might lead to higher participation, while on the other hand it requires survey managers to devote more of their time to the data collection efforts. To facilitate the decision about the length of the field period, we study how sample composition changes during the field period. Our main research question is whether a longer fielding time reduces the risk of nonresponse biases.

Methods & Data: We use data from the GESIS Panel, a probability-based mixed-mode panel of the general population in Germany that was recruited in 2013 and started the regular operation in 2014 (N(active panel)= 4,938). We analyze both the online and the offline samples, in which respondents were invited to bi-monthly online and mail surveys with a field period of two months. Drawing on information collected during the recruitment interview and the first self-administered survey in 2013, we predict response propensities and divide the panelists into three different strata. We then analyze whether these propensity strata show different response patterns over the field period. We seek to answer the question whether we are getting “more of the same” respondents (i.e., potentially higher nonresponse bias) or different respondents (i.e., potentially lower nonresponse bias) towards the end of the field period.

Results: Preliminary findings indicate that different variables predict participation in online and offline modes. Response patterns of the two high-propensity strata are similar for the online and offline modes, however, response patterns for the low-propensity strata differ over the field period between the modes. There is an indication that decreasing the length of the field period would increase potential bias in the online mode more than in the offline mode.

Added Value: This study contributes to the understanding of response patterns during the field period. Analysis of potential biases due to increasing or decreasing the fielding time can help survey practitioners determine the optimal field period for online and mail surveys.