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Table 1 Definitions of key epidemiological terms

From: Contextual factors in maternal and newborn health evaluation: a protocol applied in Nigeria, India and Ethiopia

Internal validity the extent to which the research tool is really measuring what it purports to measure [46]
External validity the extent to which the research findings can be generalised to the wider population of interest and applied to different settings [46]
Confounding a situation in which the estimate of association between an exposure and an outcome is distorted because of the association of the exposure with another factor that is also associated with the outcome. Confounding factors can be controlled for in certain analyses [47]
Effect modification variation in the effect of the exposure on an outcome across values of another factor (effect modifier). Stratification allows for visualising the effect: rather than controlling for it, the effect of the exposure on the outcome would need to be reported separately for different values of the effect modifier [47]