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There is a difference between effectiveness and efficacy studies involving family planning methods. Efficacy studies determine whether a method works as expected in specified research conditions; effectiveness studies measure how well a method works in real life. The efficacy of a family planning method is usually determined prospectively by studies conducted in a controlled environment. Effectiveness, on the other hand, is based on the unintended-pregnancy rate among a large population of users over time, usually determined retrospectively through chart reviews or surveys. Most studies of NFP are controlled studies—that is, they are efficacy rather than effectiveness studies.
Richard J. Fehring (2019) studied this question.