Sex appears in my inbox at least once a month. We all occasionally wonder if we’re normal, especially when it comes to the most private parts of our lives. This week, after I received two questions from readers about their masturbation habits (something I’ve quantified before by age, frequency and relationship status), I decided to return to the original data source to see if I had missed anything.

I had — statistics on what makes women and men reach orgasm.

In 2009, the National Survey of Sexual Health and Behavior (NSSHB) asked 1,931 U.S. adults ages 18 to 59 about their most recent sexual experience. The topline findings show that men are more likely to orgasm than women — 91 percent of men said they climaxed during their last sexual encounter, compared with 64 percent of women.

But there seems to be a perception gap, too — at least among men. Eighty-five percent of men said their partners in that recent sexual encounter had reached climax, far higher than the percentage of women who said they orgasmed. That can’t simply be explained away by saying that the men were referring to different sexual partners. Most of these sexual encounters were heterosexual — 92 percent of men and 98 percent of women said their last sexual encounter was with someone of the opposite sex. So it seems like some of those men were wrong when they said their partners had orgasmed — either their egos are causing them to overestimate, or some of those women are faking it.

The survey also asked which sexual acts people had engaged in and whether they had experienced an orgasm during that encounter. It’s worth mentioning at this point that only people who reported a sexual experience in the previous 12 months were included in the survey — presumably because it might get tricky to accurately recall sexual encounters after a while, especially the forgettable ones.

For men, the results don’t vary much — they orgasmed around 90 percent of the time regardless of which sexual acts the encounter included. But for women, there were some big differences — 64 percent of women reported orgasms in encounters that included partnered masturbation (defined in the study as “masturbating with a partner, rubbing genitals together, dry sex, or humping”), while 81 percent orgasmed during encounters in which they received oral sex.

Anal sex appears to be the hands-down winner for both sexes — 100 percent of men and 94 percent of women say they orgasmed during encounters in which they received anal sex. However, these numbers are less reliable than the others cited in the chart because only 25 men and 31 women interviewed said they received anal sex during their last sexual encounter.

That’s not to say that anal sex, or any other sex act, worked on its own. Generally speaking, the more sexual acts that were performed during the last sexual encounter, the higher the probability that respondents would say they had climaxed. By the time they experienced five sexual acts in one session, both men and women said they had orgasmed more than 89 percent of the time. I should bloody well hope so after all that effort.

Have a question you would like answered here? Send it to @MonaChalabi or dearmona@fivethirtyeight.com.