Measuring up to expectation: Cognitive bias in wrist range-of-motion measurement

STUDY DESIGN: Controlled laboratory study, cross-sectional design. BACKGROUND: The role of cognitive biases and their effect on a wide range of aspects relevant to clinical medicine has become the focus of a growing body of research, yet their effect in physical therapy is not well established. OBJECTIVES: To test whether anchoring information provided to physical therapists prior to assessment of wrist range of motion (ROM) may induce bias in the measurement. METHODS: A total of 120 physical therapists participated in the study. Participants were asked to measure passive wrist extension ROM of a 65-year-old woman with no history of injury to the upper limb using a universal goniometer. Before initiating the measurement, some participants received a clinical description, which included sham information about the patient's health history. Three groups were differentiated according to the provided clinical content: no bias (n = 38), moderate bias (n = 41), and substantial bias (n = 41). An analysis of covariance was applied to test for differences between the 3 groups while controlling for any potential sex and experience effects. RESULTS: The analysis of covariance yielded a significant group effect (P = .009), with no significant effect for sex and experience. The adjusted mean wrist ROM was 80.2° for the no-bias group, 74.5° for the moderate-bias group, and 72.4° for the substantial-bias group. Post hoc tests demonstrated significant difference only between the group with no bias and the substantial-bias group (mean difference, 7.7°; P = .009). CONCLUSION: Anchoring information was associated with differential results of an objective test. Physical therapists should increase their awareness of biases and consider employing debiasing strategies.