What does it mean when they say The Pill is “99% effective”?

Dear Cecil: I have often seen percentages used to compare the effectiveness of various methods of birth control. The pill is 99 percent effective, diaphragms are 98 percent effective, and so on. But what do these numbers mean? If the pill is 99 percent effective, does that mean it prevents pregnancy 99 times out of every 100 times a couple has intercourse, or does it mean that 99 out of 100 couples using the device will remain unexpectant over the course of a year? L.B., Los Angeles

Cecil replies:

The effectiveness percentage refers to the number of pregnancies that occur per 100 woman-years of contraceptive use. To put it another way, if 100 women use Coca-Cola as a contraceptive for a year and 20 become pregnant, the effectiveness rate of Coke is said to be 80 percent. I’m skipping some statistical subtleties here, but you get the basic idea.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.