Response

We fully accept that the studies we have included in our systematic review on decision-making capacity are highly heterogeneous. We acknowledge this clearly in the limitations section. This does indeed show that capacity is a complex problem but we attempted for the first time to get some clarity about the likely prevalence of incapacity in medical and psychiatric populations. Only 7 of our 58 included studies looked at capacity to participate in research. We accept that this is a slightly different proposition than standard treatment decisions but it is still relevant, as all the included studies tested capacity in a population to which treatment is suggested. We made sure that we only included studies which used validated tools that asked specifically about capacity for a proposed treatment. We thus excluded any general opinions on someone's capacity. We agree that it is a good idea to analyse the research capacity studies separately in a sub-analysis, and we invite the authors of the letter to submit this work. Studies on the capacity of patients with specific diagnoses have been done and are mentioned in our paper, many are included in the review.

We believe that our review has clinical value in that it gives an approximation for clinicians with regard to the level of incapacity they can expect in a variety of settings. We accept that the heterogeneity is a limiting factor, but thus far no other review has even attempted to give clinicians some guidance of the magnitude of the prevalence of incapacity.