Model specification in hierarchical meta analysis

We highlight the fact that careful consideration has to be given to the specification of the order of the hierarchy in hierarchical meta analysis. In practical applications, it can often be unclear as to which is the ‘correct’ way to specify these, and thus an arbitrary choice may be made when fitting the model. However, depending on how the model is specified, there may be important differences in the interpretation of the analysis. The points raised are highlighted using an example involving the estimation of bone density in an elderly population in a clinical setting.