Statistically safe sex

You probably think that nothing could be further apart than sex and maths, but understanding the probability and statistics behind birth control will help you make informed choices about contraceptives and keep sex safe.

Failure rates – Know the risks

There are many different types of contraceptive available, and though no method is 100% reliable, some are more so than others. Contraceptive effectiveness is measured by calculating the failure rate; the percentage of women experiencing an unintended pregnancy during the first year of use. Most methods have two different failure rates: there’s “perfect” use, for when the contraceptive always is properly applied, and “typical” use, which takes in to account mistakes like forgetting to take a pill or a broken condom.

For example, when used correctly the male condom is 98% effective, but that’s not to say there is a 2% chance of becoming pregnant every time you use one. A 2% failure rate actually means that for every 100 woman using the contraceptive method for an entire year, two will experience an unintended pregnancy. The effectiveness of some common methods are listed in the following table.



 Typical use

Perfect use

No contraceptive used



Combined oral contraceptives
AKA “the pill



Female condoms



Male condoms




Two is better than one

CondomsAlthough the statistics show that condoms are less effective than the pill, they are the only form of contraception that can prevent the spread of sexually transmitted infections (STIs). To make sure you are protected against both the risk of pregnancy and contracting an STI, you should combine condoms with another type of contraception.

Thanks to the laws of probability, using two contraceptive methods at the same time boosts their effectiveness. Remember, if two events A and B are independent, the probability of both occurring is P(A and B) = P(A) x P(B). If you use both a condom and the pill they work independently, because if one method fails it has no impact on the other. That means the failure rate of the combined condom and pill method is the product of their separate failure rates. For perfect use of the male condom and pill, that’s 0.02 x 0.003 = 0.00006, or 0.006%. In other words, the combined method is over 99.99% effective when used perfectly.

The figure for “typical” use of the combined method is also much higher than using either method on its own. With failure rates of 15% for the male condom and 8% for the pill it pays to use them together, since 0.15 x 0.08 = 0.012, or 1.2%. With this rating of 98.8% effectiveness, using the combined method is clearly much safer. No one expects you to get out a calculator before you have sex, but you should always understand the risks involved and choose the right contraceptive methods for you.