A failure rate of close to 30% is horrible.
Here is an explanation I found describing how to calculate failure rates:
Contraceptive effectiveness is usually reported as a percentage. These different effective numbers represent success in âwomen years.â So if a form of birth control is 98 percent effective, that means that, on average, 98 out of 100 women using it for one year will not get pregnant. Occasionally, contraceptive effectiveness is reported in terms of âfailure rates.â So in the above content, the same form of birth control would have a 2 percent failure rate, meaning that 2 out of 100 women using this form of birth control would get pregnant in the period of one year. You also often see two numbers reported for each form of birth control, ideal and typical. Ideal represents the level of effectiveness if used perfectly every time; typical rates take into account human failure. Hereâs a neat trick: you can compute the effectiveness of combining two forms of birth control by multiplying the numbers. So for example, if you combine a condom (12 percent typical failure rate) with spermicidal foam (21 percent typical failure rate), the failure rate plummets to 2.5 percent.