The Effectiveness Of Natural Family Planning
by John Kippley
Part 3 — USER EFFECTIVENESS
Part 1: Introduction: The Effectiveness of NFP
Part 2: Method Effectiveness
Part 3: User Effectiveness
Part 4: The Ethics of FDA and ACOG Regarding NFP
Part 5: References
In any group of people participating in any test of a method of birth control, there will be some couples who do not follow the rules. For simplicity, we will call them "chance-takers."
The term "user effectiveness" includes the pregnancies of couples who did not follow the rules — the "chance-takers."
The percentage of chance-takers will vary according to their relative motivation to avoid pregnancy and also according to the perceived difficulty of the method. Therefore birth statisticians have classified users into two categories: "limiters" and "spacers."
Limiters are those who desire no more children: for example, a couple in their mid-forties with five living children might classify themselves as limiters.
Spacers are those who consciously intend to have future children but want to delay pregnancy. Even within such a group there may be significant differences in motivation from that of delaying pregnancy a few months so birth will occur in a certain season to delaying pregnancy until an unemployed breadwinner is re-employed, etc.
Where significant differences in effectiveness rates are found between groups using essentially the same method, the differences say more about the makeup of the different groups or their training than it does about the method; where the members of two groups which are essentially the same have significant differences in effectiveness rates, the differences say something about the different methods.
The Los Angeles study was developed specifically to test for user effectiveness differences between the STM and the OM.7 Therefore the participants were randomly assigned to either side of the study, and the resulting populations were statistically very well matched.
The study population was essentially a group of spacers for two reasons: 1) no one with a serious reason to avoid pregnancy was allowed in the study; participants were warned that there might be an unplanned pregnancy rate of 25%, and they had to be willing to accept that risk to enter the study. In such a group of "spacers" it is hardly surprising that a number of couples decided to "take chances," i.e. have intercourse at a time indicated as fertile by the particular method. Thus, the STM group experienced a "user effectiveness" rate of 15 pregnancies per 100 woman years (i.e. an 85% effectiveness rate) while the OM group experienced a "user effectiveness" rate of 37 per 100 woman years (i.e. a 63% effectiveness rate). The designers of the study concluded that the difference was statistically very significant in favor of the Sympto-Thermal Method. (Significantly higher user-effectiveness rates were reported in the five nation WHO study of the OM:8 20.6 per 100 woman years according to the standard Pearl formula.)
The Fairfield study showed somewhat similar results.9 In the same study that showed a pure method surprise pregnancy Pearl rate of only 0.75, there was an overall user effectiveness rate of 5.49, still a most respectable 94.5% effectiveness rate. However when the spacers were analyzed as a separate group, the five nation average Pearl rate was 14.83 per 100 woman years — i.e., an 85% effectiveness rate.
The Austrian and German studies show a very narrow spread between method effectiveness, and that probably says that as a whole the couples in those studies were more committed to keeping the rules.
In the Roetzer study, the overall user-effectiveness Pearl rate of 0.8 (12 pregnancies in 17,026 cycles), is still within the 99% level of effectiveness.10
In the Doring study, there were 125 pregnancies in approximately 48,387 cycles, yielding a Pearl rate of 3.1 per 100 woman years, i.e. a 97% effectiveness rate.11
Conclusion 2. Studies conducted in Austria, Canada, Colombia, France, Germany, Mauritius and the United States have demonstrated a user effectiveness ranging from 85% to 99%. The evidence also strongly supports the common sense notion that the lower effectiveness rates will be found in groups which contain a significant proportion of couples who have no serious reason for postponing pregnancy.
For purposes of comparison, it should be noted that the 85% user effectiveness rate of the STM when used by spacers is equal to or higher than the user effectiveness of any other non-permanent method of birth control except the Pill.12
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