Failure rates are determined during clinical trials in smaller populations where they also survey compliance to use of the contraceptives and factors that affect compliance. So these rates are obviously not completely accurate in all populations/contexts.

The failure rate is the difference between the expected number of pregnancies (per 100 people) with no contraceptives and the expected number of pregancies (per 100 people) correctly using that contraceptive. The efficacy rate is 100 minus the failure rate.

So a failure rate of 2% means an efficacy rate of 98%.