Probabilities wrote:
The effectiveness of contraception is stated in terms of usage for 1 year. Thus, if an 18 year old male relies on a condom (which is ~ 80% effective for 1 year) then the effectiveness over 4 years of college, is then calculated as 0.80 x 0.80 x 0.80 x 0.80 = 0.41. In other words, after 4 years, it's 41% likely to prevent pregnancy and 1-0.41 = 0.59 or 59% probability of pregnancy occurring.
The effectiveness of other forms mentioned, such as birth control are similarly calculated. For the pill ~ 90% effectiveness over 1 year, it would be 0.90 x 0.90 x 0.90 x 0.90 = 0.66. Or, ~ 66% effective and 34% probability of pregnancy occurring.
For women, who spend approximately 25 years of their life with an ability to get pregnant, that would translate to an effectiveness of ~ 7% — if she used contraception. In other words, that would be 93% probability of getting pregnant, despite 25 years of precautions.
In other words, you have no idea how probability works...