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So over on Tumblr (yes, that’s still a thing that’s happening), ozymandias271 explained what “condoms are 98% effective” actually means in a recent post and it’s kinda made my brain explode.

I’ve been hearing that statistic (or other similar ones) for ages, and never concerned myself with it too closely. Given how little casual (or any kind of) sex I’ve generally been having, it wasn’t of much personal importance, and while I advocate strongly for comprehensive sex and relationships education, it should definitely be someone better-informed than me doing it. But I knew enough to know that condoms are good, and knowing how they work is also good, and if I needed more detailed data than that I’d surely be able to do the research.

But 98% always seemed oddly low. I wasn’t sure how much it was affected by issues like compliance or user error – is that remaining 2% at least partly explained by people just applying them wrong? – but taken on its face that’s actually quite a high-sounding failure rate. Do you really only have to have fifty sexual encounters involving a condom before you’ve statistically had one for which it might as well not have been there and you’re facing all the risks of unprotected sex? Given how much sex straight people on TV seem to be having, this makes it sound like unplanned pregnancies due to contraceptive ineffectiveness would be cropping up pretty regularly, and just something to be accepted as par for the course.

Anyway it turns out that’s totally not what “98% effective” means. Taking the outcome of unplanned pregnancy specifically, here’s how one website describes the effectiveness of condoms:

In one year, only two of every 100 couples who use condoms consistently and correctly will experience an unintended pregnancy—two pregnancies arising from an estimated 8,300 acts of sexual intercourse, for a 0.02 percent per-condom pregnancy rate.

98% effective doesn’t mean a condom is only doing its job in 98% of sexual encounters. It means that 98% of people using condoms for a year will avoid unplanned pregnancies in that year.

Or, assuming you’re using them correctly and having sex about as often as these statisticians imagine, the length of time the average person would have to keep having regular safe sex before encountering a condom failure isn’t fifty sexual encounters, but fifty years.

I have been massively misunderstanding this for YEARS because of what seems like REALLY UNCLEAR COMMUNICATION AND UNHELPFULLY OBSCURE PHRASING, GUYS. Seriously, I can’t be the only one who finds that a totally counter-intuitive interpretation of the “98% effective” line. Did everybody but me already have this figured out? I mean, it’s less important that I understand this than almost anyone else, but still.

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