100 women were surveyed for oral contraceptive pill usage for 24 months, out of which 10 became pregnant. Calculate the pearl index?
Ref. Foundations of Community medicine. Page.117
 
Pearl Index

It measures the contraceptive efficacy.
It is defined as the number of failures per 100 woman  years of exposure
This rate is given by the formula = Total  accidental  pregnancies * 1200 

Number of women * Number of month
 
Pearl Index = 10 × 12 × 100 / 100 × 24 = 5
2.5
5
10
20