Calculating the Failure Rate of Birth Control?!

Dear Couch,

When combining more than one method of birth control, is it accurate to multiply the individual failure rates to get the combination's failure rate? For example, the failure rate for withdrawal is approx. 1/5 and for spermicide, it's approx. 1/15. Is the failure rate of the combination 1/75 (1.3 )?


NOOO! Don't make us break out that old dusty Intro to Stats book!!! We're still in therapy over that class! You're technically right in your math. Given these FDA contraception/unwanted pregnancy rates released in May of 1997, if you used a plain latex condom in combination with a vaginal spermicidal sponge, the rate of failure would be 1/7 * 1/5, which is 1/35 or a 2.8 chance of becoming pregnant. However! Realistically, if the condom breaks it's out of the equation, so you're back to a 1/5 or 20 chance of becoming pregnant. We understand that most forms of birth control don't have the visible signs of not working like a condom, we just wanted to make it clear that in the event that you know one form of combined birth control is no longer working, then you only have the effectiveness of the other one to count on. For the best guide to choosing contraception try Ann Rose's Ultimate Birth Control Links .

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