A general overview of Mantel-Haenszel methods: applications and recent developments.

Many health research investigations are concerned with the relationship be­ tween a primary factor, such as a potentially harmful exposure, a new therapy, or an intervention, and a response variable such as disease status, level of functioning, or extent of improvement. Quite frequently, both of these variables are reported on categorical measurement scales, either nomi­ nal or ordinal , so that the reSUlting data can be displayed as observed frequencies in a two-way contingency table. However, this factor-response relationship also may be influenced by additional variables, typically referred to as intervening variables, confounding variables, effect modifiers, or covariables, because of their effects on the response variable and its relation­ ship with the primary factor. For example, the study may be conducted at several institutions involving different investigators and patient popUlations;