As student evaluation of teaching (SET) instruments are increasingly administered online, research has found that the response rates have dropped significantly. Validity concerns have necessitated research that explores student motivation for completing SETs. This study uses Vroom's [(1964). "Work and motivation" (3rd ed.). New York, NY: John Wiley & Sons] expectancy theory to frame student focus group responses regarding their motivations for completing and not completing paper and online SETs. Results show that students consider the following outcomes when deciding whether to complete SETs: (a) course improvement, (b) appropriate instructor tenure and promotion, (c) accurate instructor ratings are available to students, (d) spending reasonable amount of time on SETs, (e) retaining anonymity, (f) avoiding social scrutiny, (g) earning points and releasing grades, and (h) being a good university citizen. Results show that the lower online response rate is largely due to students' differing feelings of obligation in the 2 formats. Students also noted that in certain situations, students often answer SETs insincerely.