The AAU study that most people get the 1 in 4 number from is laughably flawed, and includes a warning from the people who composed the study that it is neither representative nor complete. To quote
HuffPo on what an accurate headline about that study would look like:
The actual Bureau of Justice statistics reflect much lower numbers. I recommend starting with Rape and Sexual Assault Victimization Among College-Age Females, 1995–2013. Their numbers are reflective of young women, both in and out of college, and it a picture roughly one order of magnitude less abhorrent than the "1 in 4" number out of the horribly misused AAU study. The other Bureau of Justice studies are, of course, important for understanding the full picture, but this one reflects the most targeted age group.
I also checked the citation behind the 1 in 6 number, and found a survey from 1998, which RAINN presents "for educational purposes only." They also said that the number of victims of rape and sexual assault has likely increased drastically, even though their Bureau of Justice sources indicate that both have been decreasing drastically. Again, I point to the college-aged women study which found a decrease of approximately 63% in the last 2 decades of rape & sexual assault.
In summary, it appears that RAINN includes two faulty and unreliable studies for the express purpose of making scary-sounding statistics for attention - literal memes, designed to spread faster & more virulently than the more accurate, yet boring information provided the BoJ studies.
Roughly 1 in 40 is frightening enough of a number without resorting to the use of painfully flawed studies.