# effectiveness of contraception

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posted on Aug, 23 2005 @ 12:18 AM
I'm so confused about what "effectiveness of contraception" means exactly.
Does it mean the chance that pregnancy will be prevented in one sex act? Or does it mean the chance that a girl will become pregnant over some period of time? Or what?

I've studied a lot of probability and statistics, I wanted to know what the chances are that a girl will become pregnant using a certain contraceptive method, and having sex a certain number of times. If 'X' is the effectiveness of a certain contraceptive, and 'n' is the number of times sex occurs, and 'P' is the probability that a girl will become pregnant once in all of those sex acts, then I found out:

P = 1 - X^n

Let's do some math: some websites claim that typical use of the pill is 97% effective, (X=0.97, ideally it could be 99.9% effective), if a girl has sex 100 times then there's a 95% chance (P=0.952) she will become pregnant at least once according to this equation. Somehow I doubt that 19 out of 20 women are becoming pregnant on the pill after having sex only 100 times. This leads me to believe that maybe the definition of effectiveness is not clearly explained.

For those who want to find the effectiveness of a combined method of contraception, you can use the following equation:

X = 1 - (1 - Y)(1 - Z)(1 - A)...

where Y,Z,A, etc. are the effectiveness of one form of contraception in the combination.

Go ahead and do some calculations of "typical" effectiveness of different methods of contraception, not "ideal" numbers. I did, and it seems to me that girls should be getting pregnant a lot.

could somebody PLEASE explain to me what effectiveness of contraception means? Or if my definition is correct then somebody needs to explain the error in my math please.

posted on Aug, 23 2005 @ 12:23 AM
Perhaps you are thinking just a bit too much. I think when birth control methods advertise the effective statement has to do with one event. If you are looking for a lifetime guarantee then your best bet is to not get involved at all.

posted on Aug, 23 2005 @ 12:24 PM
I also think you are thinking way to much on the stats here,

Simple the only 100% contraception is no sex.

But... Learning about all forms of contraception is a good thing,

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