避孕指數（珀爾指數）
為了得到有關避孕成敗相當可靠的資料，研究者需要較大的統計樣本，並且使用避孕藥具1200個月也被認為足夠達到要求了。具體來說：研究者追蹤研究100個婦女，她們進行規律的陰道性交和使用確切的避孕方法。12個月以後，在這個樣本組裏統計非意願懷孕的例數。假如兩個婦女在那一年內懷孕了，對於這種特定的避孕方法的失敗率，也就是珀爾指數為2。假如5個婦女懷了孕，珀爾指數就是5。換句話說：珀爾指數越高，避孕的失敗率就越大。反之：珀爾指數越低，所調查的避孕方法就更有效。
然而，不應該把用珀爾指數衡量的失敗率絕對化。首先，不同的資料來源會提出相異的指數數字。有的資料只是由專題研究所提供，其他資料是具有不同級差的估計值。還有，通常要在整個調查組內要做到盡可能地相一致。無論如何，避孕失敗常常不僅因為避孕方法無效，而且因為使用方法不當。由於這樣和那樣的原因，珀爾指數絕不能是一個標杆，它是一個有用的提示，是一條有關避孕狀況的總說明。不過，珀爾指數明顯的優勢，在數個因數中，當個人或夫婦為自己挑選避孕方法時，珀爾指數有其使用價值。 |
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Contraception |
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Methods
of Contraception: Success and Failure of
Contraception
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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. |
