External validity of a randomised clinical trial of temporomandibular disorders: analysis of the patients who refused to participate in research

Purpose: To assess the external validity of a randomised clinical trial (RCT) of a painful condition. Method: Consecutive patients with painful temporomandibular disorders (TMDs) were invited to participate in a clinical trial. Patients who refused to participate were compared to those who agreed to participate in this study with respect to degree of symptoms at time of presentation. Results: The patients who refused to participate had more pain, and their condition interfered more with their daily life than those who accepted the invitation to participate. Conclusion: Selection bias in RCTs of painful conditions can skew the results, and external validity should be analysed before the results are generalised.