# Probability - Contraception

With a perfect use of the condom you have a 2% chance to get pregnant each year. These figures seem surprisingly high to me. So I was wondering what the chance is to have a baby after a lifelong use (let's assume 25-35 years).

What if you use the pill? And with both the pill & condom? What with the "typical use" scenario?

• Pill | Perfect use: 0.3%
• Pill | Normal use: 8%
• Condom | Perfect use: 2%
• Condom | Normal use: 15%
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Assuming 30 years, 8.6% chance to have (at least) a baby with pills - perfect use, 92% with pills - normal use (I suppose "normal" means erratic then ;)), 45% with condoms - perfect, and 99% with condoms - normal (again, I think you're quite bad at it if you get 15% chance to get pregnant). –  Plop Mar 7 '11 at 19:36
To begin with, "a 2% chanche to get pregnant each year" seems not really meaningful. One should also specify the number of.... well, "events" per year. –  Andrea Mori Mar 7 '11 at 19:38
I suppose the 2% is an average gotten from test results. And @plop, some info on how to get the results would be nice :) –  Carra Mar 7 '11 at 21:59
There is only one way, and it is written right below. –  Plop Mar 7 '11 at 23:30

If you have a probability of failure for one year of $p$, you have a probability of no failure for one year of $1-p$. Then the chance of no failure for $n$ years is $(1-p)^n$ if you assume the chances are independent.