Objective: To quantify the amount of missing data in campus sexual assault surveys and understand how they may influence key conclusions. Methods: We systematically reviewed 40 campus sexual assault surveys conducted from 2010-2016. We 30 constructed a pseudo-population of the total population across schools, creating records proportional to the respective response rate and reported sexual assault prevalence. We then simulated the effects of 9 scenarios where the sexual assault prevalence among non-responders differed from responders. Results: The surveys represented 317,387 female undergraduates with only 77,966 (24.6%) survey responses. 35 Among responders, 20.4% reported sexual assault experience. However, sexual assault prevalence could theoretically range from 5.0 to 80.4% under extreme assumptions about prevalence in non-responders, with smaller differences observed under less extreme assumptions. Conclusions: Missing data are widespread in campus sexual assault surveys. Conclusions drawn from these 40 incomplete data are highly sensitive to assumptions about the sexual assault prevalence among non-responders.