Bagleworm posted:

(I think I covered everything!)

Birth control's effectiveness is measured by two different numbers: the typical use and the perfect use failure rate. Both of them attempt to give an idea of how many pregnancies will result from the use of a single method of birth control for a year.

Perfect use is exactly what it sounds like: use the method perfectly for a year, and this failure rate is the chance the method will fail on its own. This result usually comes from clinical trials over the first year of use.

Typical use is what an average person can be expected to see. Typical use usually comes from surveys and studies conducted in the real world, where people make mistakes. (They are occasionally estimated based on the typical use rates of similar methods) The result is calculated as a simple percentage: calculate how many women got pregnant while using this method as their only form of contraception, regardless of if they followed the method's instructions. (Typical use is usually calculated based on the first year. Some methods have lower typical use failure rates if you factor in more years!)

Typical use failure rates can include things like missing pills, breaking a condom, not pulling out if the pull out method is your only means of contraception, being late for your Depo reinjection... that sort of thing. Typical use failure rates can vary a few percentage points depending who did the study and on their sample size, but they are generally a good indicator of how easy the method is to do correctly. Should the method have a small perfect use but a high typical use failure rate, it generally indicates that it is easy to mess up.