Modelling centre effects Model C

So far, the model has taken no account of the fact that the data are recorded at different centres. It is possible that values in some centres may tend to be higher than those in other centres. Such differences could be due, for example, to differences in the techniques of personnel across centres. It is also possible that some centres/clinics may recruit patients with differing degrees of severity of hypertension (within the bounds of the study entry criteria) who could, on average, have higher or lower values of DBP. We can allow for these possibilities by adding centre effects to Model B:

Cj = the jth centre effect.

Thus, part of the residual term in Model B may now be explained by the centre effects, Cj. If there are differences between the centres, this model will have a smaller residual variance than Model B (i.e. a smaller a2). This in turn allows treatment effects to be calculated with greater accuracy.

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