The safety of each birth control method is measured in several repeated studies in which a large amount of women uses this particular method. After the study is done, it is counted how many women still got pregnant. The level of safety is usually measured in percentages. It is calculated in this way: if, eg., 100 women use the particular birth control method for one year, then the percentage shows how many of them didn’t get pregnant.
For example, hormonal contraceptives are 99.8% safe, condoms – 98%, spermicidal contraception – 60–90%, contraceptive coil – 98%, sterilization – almost 100%.
The efficiency of the method is described with Perl index, which shows the amount of women who in the end did get pregnant, even though they used the birth control method in a correct way for one year.
The efficiency that is described in this way is 0.2 for the hormonal contraceptives (pills) meaning that 0.2 out of 100 or 2 out of 1000 got pregnant, using the pills for one year).
You have to remember that the only 100% safe birth control method is abstinence from sex. And keep in mind that condoms are the only method that also assures not getting any STDs.