The method effectiveness of a particular method of family planning assumes its “perfect use.” As effectiveness rates are calculated by the number of surprise pregnancies, method effectiveness calculations only include those pregnancies that resulted from correctly following the rules of the method and the months or cycles in which the method was correctly and consistently used.
For Natural Family Planning method effectiveness, this means that the couples correctly followed and applies all of the rules. A parallel example of method effectiveness involving the birth control pill would assume that the Pill is taken exactly as prescribed every day without exception. Another example involving condoms would mean that a condom was used correctly and consistently every single time.
User effectiveness of a particular method of family planning is based on the actual practices of the couples using the method. It refers to all pregnancies occurring during a study and all months or cycles, which includes incorrect and correct application of a method and its rules.
For example, user effectiveness of the Sympto-Thermal Method of NFP would include a surprise pregnancy that resulted from a couple who engaged in the marital embrace during days when mucus was observed prior to ovulation. User effectiveness also accounts for a couple’s misunderstanding and application of the rules, and/or inaccurate recordings.
These terms are applicable to both contraception and NFP.