FBI reports consistently put the number of "unfounded" rape accusations around 8%. The average rate of unfounded reports for Index crimes is 2%. However, "unfounded" is not synonymous with false allegation. Bruce Gross of the Forensic Examiner's says that:
This statistic is almost meaningless, as many of the jurisdictions from which the FBI collects data on crime use different definitions of, or criteria for, "unfounded." That is, a report of rape might be classified as unfounded (rather than as forcible rape) if the alleged victim did not try to fight off the suspect, if the alleged perpetrator did not use physical force or a weapon of some sort, if the alleged victim did not sustain any physical injuries, or if the alleged victim and the accused had a prior sexual relationship. Similarly, a report might be deemed unfounded if there is no physical evidence or too many inconsistencies between the accuser's statement and what evidence does exist. As such, although some unfounded cases of rape may be false or fabricated, not all unfounded cases are false.