Overall the study appears of reasonable quality, however there are several study design problem areas (additional to the ones described by the authors) which may limit its external validity. Firstly, the economic question is based and heavily dependent on the estimates of effectiveness of the two regimes as judged by the studies reviewed. Unfortunately in the study little visibility is given to the limitations of the three effectiveness studies and consequently the reader is left with the uncertainty of the limitations of the economic analysis. This limitation is made worse by the absence of any explicit sensitivity analysis.
Secondly the text is not really sufficient to explain how the economic model applies to the four populations to which it refers. For example the study by Weinstein (reference 11 in the text), of unknown design, only appears in the results section and is used to justify a modelling of a 55% reduction in the incidenceof hip fractures after four years of "taking vitamin D". Two of the studies reviewed came from outside of the UK (France and Finland) and, as such, there is doubt concerning their generalisability to the UK context. Finally costs appear to be mainly acquisition ones, but little information is given about the "cost of a hip fracture" (5,000) upon which the claim of resourcing savings is made.