Welch W P
Med Care. 1985 Nov;23(11):1234-41. doi: 10.1097/00005650-198511000-00002.
As more Americans choose among insurance plans, the possibility of biased selection increases in importance. Although regression toward the mean is recognized as a common problem in evaluating social programs, it has generally been ignored in studies of biased selection. Suppose that people are included in a group simply because they had expenditures in one year $100 below the mean; that is, health status or other risk factors are not part of the selection criteria. Empirically, the expected difference in the following year is about $20 and appears to fall in each subsequent year. This pattern holds for the elderly and nonelderly. Evidence of lower pre-enrollment expenditure of prepaid group practice (PGP) enrollees can be interpreted in several ways. Under one interpretation, PGP enrollees are assumed to be a random sample conditional on pre-enrollment expenditure, such that biased selection is one fifth of estimates based on 1 year of data and one half of estimates based on 4 years of data. This article cannot resolve the issue of alternative interpretations; it only raises it.
随着越来越多的美国人在保险计划中做出选择，有偏差选择的可能性变得愈发重要。尽管均值回归在评估社会项目时被视为一个常见问题，但在有偏差选择的研究中通常被忽视。假设人们仅仅因为某一年的支出比均值低100美元就被纳入一个群体；也就是说，健康状况或其他风险因素不是选择标准的一部分。从经验上看，次年的预期差异约为20美元，并且在随后的每一年似乎都会下降。这种模式在老年人和非老年人中都成立。预付团体医疗服务（PGP）参保者预参保前支出较低的证据可以有多种解释。根据一种解释，假设PGP参保者是以预参保前支出为条件的随机样本，这样有偏差选择是基于1年数据的估计值的五分之一，是基于4年数据的估计值的二分之一。本文无法解决替代解释的问题；只是提出了这个问题。
