Abstract
Different nutritional deficiencies often coexist - both within populations and within individuals - and their interactive effects are likely to be qualitatively and quantitatively different from the summation of the independent effects of each deficiency in isolation. Thus, the generalizability of research findings on the functional consequences of specific nutrient deficiencies from one population to another is open to question if the general health and nutritional conditions of the target populations are not equivalent. However, population studies often lack information on other nutritional and health factors that have the potential of acting as effect modifiers. An analytic strategy to follow is to compare results from studies testing the same hypothesis in different population and ecological conditions. Consistency of findings using such a strategy provides insurance of external validity. This paper discusses the consistency of findings in four studies on the functional effects of iron deficiency on behavioural development and the discrepancy of the findings regarding physical growth.