# Measuring effectiveness of birth control

**URL:** <https://boards.straightdope.com/t/measuring-effectiveness-of-birth-control/81230>\
**Category:** Factual Questions\
**Created:** [September 13, 2001, 2:01am UTC](https://boards.straightdope.com/t/measuring-effectiveness-of-birth-control/81230 "2001-09-13T02:01:25Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![bizerta](https://avatars.discourse-cdn.com/v4/letter/b/3be4f8/32.png) [@bizerta](https://boards.straightdope.com/u/bizerta)\
**Post date:** [September 13, 2001, 2:01am UTC](https://boards.straightdope.com/t/measuring-effectiveness-of-birth-control/81230/1 "2001-09-13T02:01:25Z")

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A recent TV add for an oral contraceptive stated that it was 99% effective. How is that measured? I believe that the average married couple has sex about 100 times a year. Does that mean she can expect to get pregnant after a year of normal sex? Obviously not, so what do they mean: 99% effective.

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**Author:** ![Duck\_Duck\_Goose](https://avatars.discourse-cdn.com/v4/letter/d/50afbb/32.png) [@Duck\_Duck\_Goose](https://boards.straightdope.com/u/Duck_Duck_Goose)\
**Post date:** [September 13, 2001, 2:21am UTC](https://boards.straightdope.com/t/measuring-effectiveness-of-birth-control/81230/2 "2001-09-13T02:21:22Z")

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Just a WAG. When they finally tested it on human subjects, 1% of the women who took it got pregnant anyway.

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**Author:** ![Lamia](https://avatars.discourse-cdn.com/v4/letter/l/8e7dd6/32.png) [@Lamia](https://boards.straightdope.com/u/Lamia)\
**Post date:** [September 13, 2001, 2:22am UTC](https://boards.straightdope.com/t/measuring-effectiveness-of-birth-control/81230/3 "2001-09-13T02:22:35Z")

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> [@](#):
>
> \*Originally posted by bizerta \*  
> \*\*A recent TV add for an oral contraceptive stated that it was 99% effective. How is that measured? I believe that the average married couple has sex about 100 times a year. Does that mean she can expect to get pregnant after a year of normal sex? Obviously not, so what do they mean: 99% effective. \*\*

I believe the percentage applies to a year of sexual activity, not to each sexual encounter. That is, the average sexually active woman on the pill has a 1% chance of becoming pregnant within the year.

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**Author:** ![LordVor](https://avatars.discourse-cdn.com/v4/letter/l/90ced4/32.png) [@LordVor](https://boards.straightdope.com/u/LordVor)\
**Post date:** [September 13, 2001, 3:58am UTC](https://boards.straightdope.com/t/measuring-effectiveness-of-birth-control/81230/4 "2001-09-13T03:58:50Z")

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Lamia’s right, it’s the percentage of people using the contraceptive who get pregnant over the course of the year. I don’t think it adjusts for how much sex you have, but it also doesn’t adjust for people using it incorrectly. (So if you miss a day or two, and get pregnant, that still counts against the pill’s accuracy.)

-LV

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**Author:** ![Popup](https://avatars.discourse-cdn.com/v4/letter/p/94ad74/32.png) [@Popup](https://boards.straightdope.com/u/Popup)\
**Post date:** [September 13, 2001, 8:19am UTC](https://boards.straightdope.com/t/measuring-effectiveness-of-birth-control/81230/5 "2001-09-13T08:19:39Z")

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The master speaks:  
[What does it mean when they say The Pill is “99% effective”?](http://www.straightdope.com/classics/a2_216a.html)

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**Author:** ![KneadToKnow](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kneadtoknow/32/3999_2.png) [@KneadToKnow](https://boards.straightdope.com/u/KneadToKnow)\
**Post date:** [September 13, 2001, 9:51am UTC](https://boards.straightdope.com/t/measuring-effectiveness-of-birth-control/81230/6 "2001-09-13T09:51:15Z")

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Good work and welcome aboard, **tc**.

My first car was nicknamed **tc**. So I’ll be watching your career here with no small amount of interest. 🙂