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.