Statistics notes: Interaction 3: How to examine heterogeneity

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Abstract
In preceding Statistics Notes we introduced the concept of interaction1 and explained why a common approach to the assessment of interaction is incorrect.2 In this note we give details of the correct approach using the same two examples. In a study of the effect of maternal vitamin D supplementation on neonatal serum calcium concentrations3 the researchers were interested in the possible difference between the effect of supplementation on breast and bottle fed babies. We define the treatment effect in each feeding group to be the difference in the mean serum calcium concentration of babies receiving supplements and those receiving placebo in …