The effectiveness of contraception methods is typically reported as two percentages: typical use and perfect use.
Typical use is the measure of how the average couple is likely to use the method, and takes into account incorrect or inadequate application. For example: an inside-out condom, or a missed day of birth control pills.
Perfect use assumes the couples being studies used the contraceptive method exactly as directed, without errors.
How are participants partitioned into these two groups?
Is there typically one large study, and participants self-report if they did not meet perfect use?
Are two groups formed prior to the study's start, and given differing levels of instruction?
Or something else?