Greenland S, Morgenstern H
Division of Epidemiology, UCLA School of Public Health 90024.
Int J Epidemiol. 1989 Mar;18(1):269-74. doi: 10.1093/ije/18.1.269.
Ecological bias is sometimes attributed to confounding by the group variable (ie the variable used to define the ecological groups), or to risk factors associated with the group variable. We show that the group variable need not be a confounder (in the strict epidemiological sense) for ecological bias to occur: effect modification can lead to profound ecological bias, whether or not the group variable or the effect modifier are independent risk factors. Furthermore, an extraneous risk factor need not be associated with the study variable at the individual level in order to produce ecological bias. Thus the conditions for the production of ecological bias by a covariate are much broader than the conditions for the production of individual-level confounding by a covariate. We also show that standardization or ecological control of variables responsible for ecological bias are generally insufficient to remove such bias.
生态偏倚有时被归因于群组变量（即用于定义生态群组的变量）造成的混杂，或者归因于与群组变量相关的风险因素。我们表明，生态偏倚的发生并不一定要求群组变量是一个混杂因素（严格的流行病学意义上）：效应修饰可导致严重的生态偏倚，无论群组变量或效应修饰因素是否为独立风险因素。此外，一个外部风险因素并不一定需要在个体层面与研究变量相关联才能产生生态偏倚。因此，协变量产生生态偏倚的条件比协变量产生个体层面混杂的条件要宽泛得多。我们还表明，对造成生态偏倚的变量进行标准化或生态控制通常不足以消除此类偏倚。
