Understanding The Numbers: How to Compare NFP Methods

Our Research page cites several different NFP studies. Some of these studies are looking at things like “typical use failure rates” and “perfect use failure rates” of different NFP or contraception methods, but what does that even mean? Let’s break it down so you can understand how the methods compare to each other.

Perfect Use Failure Rate

The first concept to understand is the “perfect use failure rate” of a method of birth control. When we talk about the perfect use failure rate, we’re asking, “If 100 people used this method to try to avoid pregnancy, how many would get pregnant in the first year?” You can look at this number over any period of time, but looking at it on a yearly basis is common. Ideally, the number would be 0, but we all know that no method of birth control is completely perfect. For example, the perfect use failure rate of condoms is about 2%. That is, 2 out of 100 people using condoms to avoid pregnancy are likely to get pregnant in the first year even if they do everything correctly.

Typical Use Failure Rate

Now that we understand the perfect use failure rate, let’s talk about “typical use failure rate”. The typical use failure rate means almost the same thing as the perfect use failure rate – it’s about how many people out of 100 will have an unintended pregnancy in the first year. The difference is that the typical use failure rate looks at “typical use” rather than “perfect use”. The best way to explain this is with an example. “The pill” has a perfect use failure rate of 0.3%. That is, only 3 out of 1,000 people are likely to get pregnant in the first year while using the pill perfectly. But we also know that almost nobody uses the pill perfectly. People forget to take it, they miss a day, life happens. When we take that into account, we get the typical use failure rate, which is about 9% for the pill. Out of 100 people using the pill, 9 are likely to get pregnant in the first year with typical use.

NFP Methods

How do perfect and typical use apply to NFP? Well, NFP methods of avoiding pregnancy also have perfect and typical use failure rates. While practicing NFP, couples sometimes forget to record, or get confused about the rules, or make some other mistake, and those mistakes account for the differences between perfect and typical use.

Look at the Numbers

Now that we understand the numbers, let’s look at some. Contraceptive failure in the United States contains a convenient table showing perfect and typical use failure rates for different methods of contraception. We can see there that the typical use failure rate for male condoms is 18%, and typical use failure rate for the pill is 9%. Now, let’s compare those numbers to some NFP methods. The Marquette Model: An Evidenced-based Electronic Hormonal Fertility Monitor Aided Method of Family Planning (on the last page) shows that the typical use failure rate of the Marquette Method varies from study to study but seems to be in the 9-13% range. So, with typical use to avoid pregnancy, we can expect the Marquette Method to perform better than condoms and on par with or slightly worse than the pill.