Dupont W D
Stat Med. 1986 Nov-Dec;5(6):629-35. doi: 10.1002/sim.4780050610.
The two tailed Fisher's exact P value is extremely sensitive to small perturbations in 2 x 2 contingency tables. An example indicates that a 1 per cent increase in the denominator of one treatment group results in a 32 per cent drop in the exact P value, but a mere 0.1 per cent decrease in the treatment success rate. This is equivalent to the increase in significance obtained by a 20 per cent increase in the sample size of both treatments without changing the observed success rates. This drop results from small changes in the probabilities of unobserved events. A systematic evaluation of 920 pairs of similar contingency tables shows that these fluctuations occur frequently over a wide range of sample sizes and significance levels. Doubling the one tailed exact P value provides a more consistent measure of inferential strength. We discuss various chi-squared continuity corrections.
双侧费舍尔精确P值对2×2列联表中的微小扰动极为敏感。一个例子表明，一个治疗组分母增加1%会导致精确P值下降32%，但治疗成功率仅下降0.1%。这相当于在不改变观察到的成功率的情况下，两种治疗的样本量都增加20%所获得的显著性增加。这种下降是由未观察到事件概率的微小变化导致的。对920对相似列联表的系统评估表明，这些波动在广泛的样本量和显著性水平范围内经常发生。将单侧精确P值翻倍可提供更一致的推断强度度量。我们讨论了各种卡方连续性校正。
