How is the efficacy of a contraceptive measured?
Efficacy of a contraceptive method is measured by two methods:
1. Pearl index
2. Life table analysis
What is "Pearl index”?
Pearl index is used for measuring the failure rate of a contraceptive. It is defined as the number of contraception failures per 100 woman years (HWY) of exposure. It is calculated by the formula:
Failure rate per HWY = Total accidental pregnancies/Total months of exposure × 1200
• The numerator "total accidental pregnancies” includes all unwanted pregnancies irrespective of the outcome, i.e.,
- Stillbirth or
- Still not terminated
• The denominator "total months of exposure” is obtained by deduction of the following from the total period of review:
- 10 months for each full-term pregnancy
- 4 months for each abortion
• "1200” is the number of months in 100 years.
• A minimum of 600 months of exposure is necessary for obtaining a valid "failure rate” value.
What is the drawback of Pearl index and how does ‘life table analysis address it?
The drawback of the Pearl index is that it assumes a constant failure rate over time. This assumption is incorrect due to two reasons:
1. The most fertile couples will get pregnant in the beginning of the study and will no longer be counted in the denominator. Couples remaining later in the study, are, on average, of lower fertility.
2. With most birth control methods the effectiveness increases with experience. The longer a couple is in the study, the better it gets at using the method. So the longer the study length, the lower the Pearl Index will be. Hence comparisons of Pearl Indexes from studies of different lengths cannot be accurate
‘Life table analysis’ calculates cumulative failure rates over a specified timeframe. They present failure rate as number of pregnancies per 100 women years, standardized by yearly cut-off points (usually 1, 3 or 5y) e.g. “life-table rates for long-acting hormonal methods were reported as 0–0.6 per 100 HWY at one year”
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