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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?

Pregnancy chances after one year of use:

  • 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
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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
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1 Answer

up vote 5 down vote accepted

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.

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So let me try. The chance of having no baby is 0.997. The chance of not having a baby 30 years in a row is (0.997)^30 or 91.38%? Or a 8.62% of having a baby when taking the pill? Also, how do you calculate it for using both the pill & condom? –  Carra Mar 7 '11 at 22:11
    
@Carra: you are right. For combinations, you need some model for how to combine the probabilities. You might think the probability is then .02*.003=.06% as both options have to fail together. –  Ross Millikan Mar 7 '11 at 22:23
    
Thanks for the help. Looks like a safe bet with a 0.2% chance after 30 years. –  Carra Mar 7 '11 at 22:28
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