避孕指数（珀尔指数）
为了得到有关避孕成败相当可靠的数据，研究者需要较大的统计样本，并且使用避孕药具1200个月也被认为足够达到要求了。具体来说：研究者追踪研究100个妇女，她们进行规律的阴道性交和使用确切的避孕方法。12个月以后，在这个样本组里统计非意愿怀孕的例数。假如两个妇女在那一年内怀孕了，对于这种特定的避孕方法的失败率，也就是珀尔指数为2。假如5个妇女怀了孕，珀尔指数就是5。换句话说：珀尔指数越高，避孕的失败率就越大。反之：珀尔指数越低，所调查的避孕方法就更有效。 
然而，不应该把用珀尔指数衡量的失败率绝对化。首先，不同的数据来源会提出相异的指数数字。有的数据只是由专题研究所提供，其它数据是具有不同级差的估计值。还有，通常要在整个调查组内要做到尽可能地相一致。无论如何，避孕失败常常不仅因为避孕方法无效，而且因为使用方法不当。由于这样和那样的原因，珀尔指数绝不能是一个标杆，它是一个有用的提示，是一条有关避孕状况的总说明。不过，珀尔指数明显的优势，在数个因数中，当个人或夫妇为自己挑选避孕方法时，珀尔指数有其使用价值。

Contraception

Methods of Contraception: Success and Failure of Contraception

The Pearl Index
In order to arrive at some reliable figures about the success or failure of contraceptive methods, one needs a large sample, and for this 1200 months of contraceptive use are considered sufficient. In practice: One follows 100 women who have regular vaginal intercourse and who use a certain method of contraception. After 12 months one counts the unwanted pregnancies within this group. If two of the women have become pregnant in that one year, the failure rate for this particular method, i.e. the Pearl index, is 2. If five of the women have become pregnant, the Pearl index is 5. In other words: The higher the Pearl index, the greater the failure rate. Conversely: The lower the Pearl index, the more effective is the contraceptive method in question.
However, the failure rates measured by the Pearl index should not be considered absolute. First of all, different sources provide different index numbers. Some are simply provided by the industry, others are estimates with various ranges. Still, there is usually some agreement on the overall ranking. In any case, contraception often fails not because the method is ineffective, but because it is not used properly. For these and other reasons, the Pearl index can never be more than a guide post, a helpful hint, a piece of general information. Nevertheless, when seen in this light, it has its uses as one among several factors when individuals or couples select a contraceptive method for themselves.