Patients’ preferences are important. Where treatment choices have different and well understood outcomes, what matters most when deciding which treatment is best is the patient’s preference. The intriguing question is whether strong treatment preferences can also affect outcomes—that is, can the preferences themselves exert a therapeutic effect over and above the anticipated effects of compliance and attrition, and if so, by how much? We know very little about this area. In the linked study (doi:10.1136/bmj.a1864), the Preference Collaborative Review Group reviews the effects of patient preferences on attrition and outcomes in trials of patients’ preferences using a meta-analysis of individual patient data.1

We know that the placebo effect can be profound. Its most plausible mechanism of action involves belief in an effect, and this must relate strongly to preferences.2 But a double blind trial of placebo versus nothing is needed to identify a placebo effect. Some meta-analyses have shown a placebo effect,3 but more convincing evidence comes from observation rather than …