The efficacy of contraception methods is typically given as two percentages: typical use, and perfect use.
For both of these, is the denominator of the percentage the number of couples who used the contraception method, or the number of instances of sexual intercourse during which it was used?
In other words, if a contraceptive is 98% effective, does that mean:
- 2% of couples who used this method (any number of times) got pregnant, or
- Of all the individual instances of sexual intercourse during which this method was used, 2% of them resulted in pregnancy?