The Bureau of Justice Statistics' (BJS) National Crime Victimization Survey (NCVS) measures the rates at which Americans are victims of crimes, including rape and sexual assault, but there is concern that rape and sexual assault are undercounted on this survey. BJS asked the National Research Council to investigate this issue and recommend best practices for measuring rape and sexual assault on their household surveys. Estimating the Incidence of Rape and Sexual Assault concludes that it is likely that the NCVS is undercounting rape and sexual assault. The most accurate counts of rape and sexual assault cannot be achieved without measuring them separately from other victimizations, the report says. It recommends that BJS develop a separate survey for measuring rape and sexual assault. The new survey should more precisely define ambiguous words such as "rape," give more privacy to respondents, and take other steps that would improve the accuracy of responses. Estimating the Incidence of Rape and Sexual Assault takes a fresh look at the problem of measuring incidents of rape and sexual assault from the criminal justice perspective. This report examines issues such as the legal definitions in use by the states for these crimes, best methods for representing the definitions in survey instruments so that their meaning is clear to respondents, and best methods for obtaining as complete reporting as possible of these crimes in surveys, including methods whereby respondents may report anonymously. Rape and sexual assault are among the most injurious crimes a person can inflict on another. The effects are devastating, extending beyond the initial victimization to consequences such as unwanted pregnancy, sexually transmitted infections, sleep and eating disorders, and other emotional and physical problems. Understanding the frequency and context under which rape and sexual assault are committed is vital in directing resources for law enforcement and support for victims. These data can influence public health and mental health policies and help identify interventions that will reduce the risk of future attacks. Sadly, accurate information about the extent of sexual assault and rape is difficult to obtain because most of these crimes go unreported to police. Estimating the Incidence of Rape and Sexual Assault focuses on methodology and vehicles used to measure rape and sexual assaults, reviews potential sources of error within the NCVS survey, and assesses the training and monitoring of interviewers in an effort to improve reporting of these crimes. Table of Contents Front Matter Summary 1 Introduction 2 Legal Definitions and Context 3 Data from Law Enforcement Agencies 4 National Crime Victimization Survey 5 Selected Other Surveys on Rape and Sexual Assault 6 Comparison of Rape and Sexual Assault Across Data Sources 7 Potential Sources of Error in the NCVS: Sampling, Frame, and Processing 8 Potential Sources of Error: Nonresponse, Specification, and Measurement 9 Synopsis of Potential Errors in the National Crime Victimization Survey 10 New Directions for Measuring Rape and Sexual Assault References Appendix A: Need for the Study Appendix B: Workshop and Public Meetings: Agendas and Participants Appendix C: Links to Questionnaires of the National Crime Victimization Survey Appendix D: Selected Surveys Measuring Rape: An Overview Appendix E: Statistical Rationale Behind Some Initial Findings on the Relative Statistical Plausibility of a Multiple-Frame Approach to Estimating the Victimization Rate of Rape and Sexual Assault Appendix F: Biographical Sketches of Panel Members and Staff Committee on National Statistics