Importance of internal and external validity in clinical research: Comment on the Article by Fatima et al.

In a recently published cohort study, Fatima et al. showed that the higher health assessment questionnaire (HAQ) score was associated with higher mortality among early rheumatoid arthritis (RA) patients.1 This study is prominent because the authors focused on early RA patients' functional disability measured by HAQ, which is more strongly associated with mortality than disease activity and joint damage in patients with established RA.2.