Not arguing for contraception, but I don’t think you can say the condom works 15% of the time that way.
The statistics is something like:
-in one year 2 out of 100 couples using condoms will experience a pregnancy.
-85 out of 100 couples using no contraception at all will get pregnant.
So by using condoms 83/100 couples who would have gotten pregnant otherwise, did not.
But this doesn’t mean it works x% of the time. The statistics are done
for a whole year, for 100 couples.
Assume for convenience that there are 10 days in a month when a woman can get pregnant, and that each of those 100 couples have sex on every fertile day in the month.
So that means 120 fertile days a year. That means 100 * 120 = 12000 fertile sex acts by
100 couples.
Out of all those, 2 pregnancies. So that’s 2/12000 * 100% = 0.017% for
one fertile sex act with a condom probability of getting pregnant. (And if you s one couple have 120 fertile sex acts in a year, that turns 2.04% per year)
I don’t think you can do the same calculation for the couples using no contraception, because since 85/100 get pregnant we don’t know if they get pregnant on the first time they have sex or the last. And once you get pregnant you can’t get pregnant again while still being pregnant. This issue is still there with the condom users, but since there’s only 2 of them that get pregnant, it doesn’t have a big effect on the number of fertile sex acts they have.