According to a report from the Metropolitan Police Department, reported rapes in Washington, D.C. have remained pretty constant over the past three years:
Reported rapes in 2006: 182
Reported rapes in 2007: 192
Reported rapes in 2008: 186
But how many rapes actually occurred in the District last year? It's impossible to say.
A 1992 report from the National Victim Center (Rape in America: A Report to the Nation) suggests that only one out of every six rapes are reported to authorities. The reason for that can be pretty well explained by another statistic from the report: "80% of the girls and women who were raped were victimized by someone they knew."
Let's assume that the National Victim Center has got it's shit together, and multiply the reported rapes by six to estimate a more realistic figure:
Estimated rapes in 2006: 1,092
Estimated rapes in 2007: 1,152
Estimated rapes in 2008: 1,116
We're getting closer now—except for the fact that records of "reported rapes" don't even reflect all of the rapes actually reported to the police.
After a crime is reported to the police, it can later be "unfounded" and purged from the numbers game. Crimes are supposed to be deemed "unfounded" if the report is found to be fraudulent. But since police departments self-report their crime data to the FBI, there's no filter to tell whether an "unfounded" case is truly false, or if they are "merely stuck or inactive" due to lack of evidence or inability to locate a suspect. Unsurprisingly, due to the he-said-she-said difficulty of prosecuting rapists, rape cases are more likely to be "unfounded" than any other crime.
MPD doesn't report how many of its reported rape cases have been "unfounded"—those cases are simply subtracted from the number of reported cases. The subtraction occurs "in the month in which [the case is] unfounded, regardless of the date the crime was committed." So in some months, more cases are unfounded than are reported: in August of 2006, MPD's Second District reported negative one rape.
So how many women are raped in D.C. every year? Two hundred? One thousand? Some negative figure? MPD's reports aren't terribly illuminating.