CNR-IBIM, Clinical Epidemiology and Physiopathology of Renal Diseases and Hypertension of Reggio Calabria, Reggio Calabria, Italy.
Nephron Clin Pract. 2010;116(4):c317-21. doi: 10.1159/000319590. Epub 2010 Jul 28.
Stratification allows to control for confounding by creating two or more categories or subgroups in which the confounding variable either does not vary or does not vary very much. The Mantel-Haenszel formula is applied in cohort and in case-control studies to calculate an overall, unconfounded, effect estimate of a given exposure for a specific outcome by combining stratum-specific relative risks (RR) or odds ratios (OR). Stratum-specific RRs or ORs are calculated within each stratum of the confounding variable and compared with the corresponding effect estimates in the whole group (that is, with the unstratified RR or OR). The use of the Mantel-Haenszel formula presents some limitations: (1) if there is more than a single confounder, the application of this formula is laborious and demands a relatively large sample size, and (2) this method requires continuous confounders to be constrained into a limited number of categories thus potentially generating residual confounding (a phenomenon particularly relevant when the variable is categorized into few strata). In the stratified analysis, residual confounding can be minimized by increasing the number of strata, a possibility strictly dependent on sample size.
分层允许通过创建两个或更多类别或亚组来控制混杂，在这些类别或亚组中，混杂变量要么不变化，要么变化不大。Mantel-Haenszel 公式适用于队列研究和病例对照研究，通过组合特定结局的特定暴露的分层特异性相对风险（RR）或比值比（OR），计算给定暴露的总体、无混杂的效应估计值。在混杂变量的每个分层内计算分层特异性 RR 或 OR，并将其与整个组（即无分层 RR 或 OR）的相应效应估计值进行比较。Mantel-Haenszel 公式的使用存在一些限制：（1）如果存在多个混杂因素，应用此公式繁琐且需要相对较大的样本量，（2）此方法要求将连续混杂因素约束为有限数量的类别，从而潜在地产生残余混杂（当变量分类为少数几个层次时，这是一个特别相关的现象）。在分层分析中，可以通过增加分层的数量来最小化残余混杂，这种可能性严格依赖于样本量。
