Robert T. Kambic, M.S.H.
In an article I co-authored with Theresa Notare, I estimated the number of people who were taught NFP in the US in a given year(1). This was done by gathering simple data and following statistical procedures. Similarly, other data collection and analysis allows scientists to estimate the numbers of people using NFP both in this country and in other countries, and to estimate the effectiveness with which they use the methods. The purpose of this short article is to describe some scientific methods used to estimate numbers of users and to tell NFP advocates what to look for when reading such information both in scientific journals and in newspapers.
There are two kinds of projects used to collect data on NFP, surveys and studies. A study is a program of data collection that focuses on a specific question or subgroup of a population; reports of NFP effectiveness are most often studies. A study might ask, "Of the couples who are taught NFP next year in our clinic, how many become pregnant within twelve months?" The information published by Ms. Notare and I was a study. This article is going to focus on surveys.
A survey is a one time interview of a representative sample of a large population of people, usually with a grand objective; the U.S. census is a survey. Questions about NFP may be one element of a survey. As an example, a survey might ask, "What is the percentage of our population using contraception, and what kinds of methods do they use?" A commonly used term for the percentage of the population using contraception is "contraceptive prevalence." Prevalence means the percentage of people in, say 1994, who have a condition, or who are doing something, here, using contraception. We say, "the contraceptive prevalence in the U.S. in 1994 is 70 percent." The prevalence of NFP use in the U.S. in 1994 is about 4 percent.
We find information about contraceptive prevalence in the U.S., in the National Survey of Family Growth (NSFG). It is a survey taken in 1977, 1983, and 1987 that, among other things, has asked about family planning use. Internationally, there have been two kinds of recent surveys that have provided information on NFP and contraceptive prevalence. The first is the Contraceptive Prevalence Surveys (CPS) and the second are the Demographic and Health Surveys (DHS).
Although surveys are one time questionnaires, the questionnaires may be long and involved as they seek to gather data on the age, sex, religion, family planning intention, economic status, and more obscure social and psychological attributes of respondents. It is in the design of questionnaires that the first problems for NFP arise. Researchers of early surveys on family planning and contraception chose to ask only about what they termed "modern methods" of contraception. By this they generally meant the pill, IUD, and sterilization. They would then combine barrier methods together and finally classify all other methods together as "traditional" or "other" methods. At times you might see "rhythm and traditional methods." Traditional methods sometimes could mean folk and superstition. I think this was done to discredit and disparage NFP and rhythm. Historical studies that lump NFP with "traditional" methods are useless for NFP analysis.
With pressure from NFP advocates and scientific work at centers like Johns Hopkins proving the credibility of NFP, survey designers had to take better account of NFP. More recent surveys such as the CPS and DHS asked about NFP in terms of "periodic abstinence" or "rhythm." The problem here of course is that no distinction was made between modern NFP, true calendar rhythm, and a "folk" calendar rhythm such as 10-10-10. However, data from such surveys at least provides some indication of the numbers and percentages of those interested in NFP. Some recent surveys have tried to discriminate between those taught NFP in clinics and those using rhythm on their own. One such survey was the Mauritius CPS. Such a survey gives a real look at the impact of the NFP program in Mauritius on the use of family planning in that country because we can identify the percent of family planning users who have been taught by a funded program.
When comparing the reports of different surveys, a critical element is to be sure that the base populations are similar. For example, one country will say that 10% of women are using NFP, and another might say that, of those women using family planning, 10% are using NFP. Both percentages are equal to ten, but 10 percent of what number? Ten percent of what base population?
Most surveys of family planning will be of women. The total number of women in the population is the best place to begin an analysis. We first reduce the total number of women to the number of women in the fertile age group, those women able to become pregnant. The fertile age group varies from survey to survey from a lower range of 15 to 20 years old, to a higher range of 40 to 45. We can find surveys reporting on the number of women 15 to 40, and others, 20 to 45. Surveys look at women in five year age groups of 15 to 19, 20 to 24, etc. Therefore surveys report on five age groups.
Next, for women of reproductive age, we carefully define the mutually exclusive groups of family planning users. The first group might be those not in a sexual relationship - they don't need family planning. Older studies handled this by defining the study group as "married women" to exclude women not in a sexual relationship. In the 1980s and 90s, this was changed to "women in union." Once we identify women in a sexual relationship, we exclude those pregnant, trying to become pregnant, breastfeeding, and non contraceptively sterile. We finally have our population of women who are in a sexual relationship and do not want to become pregnant. These are the women we question about the family planning methods they are using. Of course, the other women are noted and counted according to the rules of the survey.
Finally, besides age those surveyed may be further subdivided into categories such as race, religion, parity, etc. Within each subcategory of, for example, white Catholic women aged 25 to 29, we will want to know how many are sterilized, using the pill, using barriers, using NFP, and not using any method. If women are subcategorized into five age groups and two racial groups, we have ten categories of women for whom we want information on five classes of method use. The result is 50 subgroups of women. As more questions are added to the survey more women are required to complete the survey to insure that adequate numbers of women are in each cell.
Survey results often report a margin of error. The error term means that if the survey was repeated with different samples of respondents, the results would be within the margin of the error. If a survey says that NFP use is 4 percent in the U.S. and the margin of error is 1 percent we expect another similar survey to say that NFP use in the U.S. is three, four, or five percent but not two or six or another percent. Reported differences outside the margin of error are what scientists call statistically significant differences. Significant results require explanation to account for the differences.
When reading about survey results for NFP we should understand at least the following: the population represented by the survey, the denominator used to calculate percentages, and the error terms reported in the survey.
This table shows some of the most recent reports of NFP prevalence in 7 countries (2). The report is for currently married women, or women in union. Shown for comparison is the number of women using no method and the number using the pill. These data are representative of NFP use around the world. It is clear that very few couples in any country use NFP; Sri Lanka has one of the highest rates of use with 15 percent of women using it. I conclude two things from these data: the first is that NFP has suffered the disdain of population and family planning programs. They are not interested in it, discourage its use, and provide no money for it. In my opinion, more couples would use it if they were given the choice. My second conclusion is that there is much to be done in every country to promote the use of NFP. This promotion effort must be led by those most interested in NFP, most notably Catholics.
- R. T. Kambic & T. Notare, "Roman Catholic Church-Sponsored Natural Family Planning Services in the United States." Advances in Contraception 1994;10:85-92.
- Iqbal Shah, "Comparative Analysis of Contraceptive Method Choice," in Demographic and Health Surveys World Conference, Proceedings, Volume I, IRD/Macro International, Columbia, Maryland 1991.
Robert T. Kambic, MSH, is a research associate in the Department of Population Dynamics at Johns Hopkins University, MD. Mr. Kambic has published extensively on NFP in scientific journals.
This article first appeared in the Winter 1995 issue of NFP Diocesan Activity Report (Vol. 6 No. 1, p.8).
Copyright © 1999, Diocesan Development Program for Natural Family Planning, United States Conference of Catholic Bishops. The text and illustrations may be reproduced in whole without alteration or change by Catholic dioceses, parishes, schools, organizations, and newspapers, provide such reprints include the following notice:
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