The measurement of rape and sexual assault presents many challenges. Victims may not be willing to reveal or share their experiences with an interviewer. The level and type of sexual violence reported by victims is sensitive to a variety of factors related to the interview process, including how items are worded, what definitions are used, and the data collection mode. In addition, the legal definitions of rape and sexual assault vary across jurisdictions. While the change in the rape or sexual assault rate from year to year may be significantly different, care should be taken in interpreting this change because the estimates of rape or sexual assault are based on a small number of cases reported to the survey. Therefore, small absolute changes and fluctuations in the rates of victimization can result in larger year-to-year change estimates. BJS has initiated projects to identify, develop, and test the best methods for collecting self-report data on rape and sexual assault. For more information on BJS's active research program on the collection of rape and sexual assault data, see Measuring Rape and Sexual Assault.