The Pearl Index (Pl) compares the statistical failure rates of different methods of birth control and is the most common way in clinical trials to measure the effectiveness of contraceptives.

The higher the Pl is, the more likely it is that an accidental pregnancy will occur while using the method.

• Spermicide - 28%
• Fertility awareness - 24%
• Sponge - 22-24%
• Withdrawal - 22%
• Female condom - 21%
• Male condom - 18%
• Diaphragm - 12%
• Ring - 9%
• Patch - 9%
• Contraceptive pill - 9%
• Shot - 6%
• IUD copper T - 0.8%
• IUD LNG - 0.2%
• Implant - 0.05%

For permanent birth control methods, a vasectomy has a 0.15% Pl, while female sterilization has 0.5%.

Pearl rates are dependent upon proper use of the birth control method and on ethnicity as well. In particular, research has found that Spanish women with previous pregnancies and no prior use of hormonal contraceptives may have higher PIs.