What does the 99(.5)% of the efficiency of the contraception pill exactly mean?

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Hi there! I was wondering if this means that if you have sex 100 times, theoretically, you would get pregnant once? Or does it mean that out of the 100 times you would hypothetically get pregnant, you actually get pregnant once. Because the two make a big difference so if someone knows please let me know. Thanks! 🙂

Edit: Thanks everyone for the great answers!

In: Biology
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It’s a statistic…essentially it means that EACH time you have sex using that contraceptive device, there is a 1 in 200 shot of a failure of the device to prevent a pregnancy.

99 percent effective means that if one hundred sexually active women used the pill for a year, without using other forms of birth control, one of those women would get pregnant that year.

Birth control efficacy is measured in terms of percent of couples in 1 year who get pregnant

If a birth control is listed as 99% effective that means that if you start with 100 couples, using the product as described, one of them will be get pregnant over the course of the year. It doesn’t try to adjust for variations in going at it or variations in fertility, its generally a sample of a couple thousand couples so it all washes out into an average

The numbers are more useful for comparing types of contraceptives than for assessing whether you’re likely to get pregnant while on it (its like a big roulette wheel, you don’t know if you’ll be the “winner”). If one method is 90% effective and the other is 99% then its actually a 10x reduction in pregnancy risk.

Typically, in those studies, when they say 99% effective, they mean “We studied 100 women for a year who were using this pill, and 1 out of those 100 became pregnant.” Sometimes they’ll have two different figures – one with perfect use, where the subjects followed every direction, and one with “typical” use, where people sometimes forgot to take their pill on the proper day.

In any case, it’s about averages over a yearlong period, rather than number of times having sex.

I think they have to say that just in case it fails you can’t sue them, because you were that one percenter.