Morgenstern H
Department of Epidemiology, University of California, Los Angeles, School of Public Health, USA.
Annu Rev Public Health. 1995;16:61-81. doi: 10.1146/annurev.pu.16.050195.000425.
An ecologic study focuses on the comparison of groups, rather than individuals; thus, individual-level data are missing on the joint distribution of variables within groups. Variables in an ecologic analysis may be aggregate measures, environmental measures, or global measures. The purpose of an ecologic analysis may be to make biologic inferences about effects on individual risks or to make ecologic inferences about effects on group rates. Ecologic study designs may be classified on two dimensions: (a) whether the primary group is measured (exploratory vs analytic study); and (b) whether subjects are grouped by place (multiple-group study), by time (time-trend study), or by place and time (mixed study). Despite several practical advantages of ecologic studies, there are many methodologic problems that severely limit causal inference, including ecologic and cross-level bias, problems of confounder control, within-group misclassification, lack of adequate data, temporal ambiguity, collinearity, and migration across groups.
生态学研究侧重于群体而非个体的比较；因此，关于群体内变量的联合分布缺少个体层面的数据。生态学分析中的变量可能是汇总指标、环境指标或总体指标。生态学分析的目的可能是对个体风险的影响进行生物学推断，或对群体发病率的影响进行生态学推断。生态学研究设计可从两个维度进行分类：(a) 是否测量主要群体（探索性研究与分析性研究）；以及(b) 受试者是按地点分组（多组研究）、按时间分组（时间趋势研究）还是按地点和时间分组（混合研究）。尽管生态学研究有几个实际优势，但也存在许多严重限制因果推断的方法学问题，包括生态学和跨层次偏倚、混杂因素控制问题、组内错误分类、数据不足、时间模糊性、共线性以及群体间迁移。
