Perfect use refers to the use of a given contraceptive exactly as prescribed or indicated. When a person follows all instructions for a contraceptive at all times, their contraceptive use is said to be perfect use. In real life, however, contraception is often not used at all times or exactly as indicated. This is called typical use.
Researchers study the effectiveness of a contraceptive method by looking at how many people get pregnant when they use this method according to perfect use and typical use over the course of a year.
Contraceptives are most effective at preventing pregnancy when they are used exactly as indicated. For example, condoms are 98% effective against pregnancy under perfect use conditions, but only 85% effective under typical use conditions.
Perfect use and typical use figures are not typically used to express a contraceptive's effectiveness at preventing the spread of STIs.