In the study evaluated in this report, the authors asked what effect survey length has on student non-response rates to individual items on IDEA's "Diagnostic Feedback" (DF) and "Learning Essentials" (LE) forms. The approach was to analyze individual student ratings of classes contained in the 2015-2016 IDEA-CL database. Specifically, the authors computed item non-response rates in individual ratings of 298,209 students enrolled in 36,629 classes who used the DF, and 39,899 enrolled in 5,120 classes who responded to the LE. Tables 1 and 2 in this report show item response rates on the 41-item DF and the 19-item LE. The last item on each survey is an open-ended question for students to write comments.