Threats to the Internal Validity of Spinal Surgery Outcome Assessment: Recalibration Response Shift or Implicit Theories of Change?

A recalibration response shift will cause the patient to think about a self-report measure's response options differently after a health state change. Commonly assessed using the retrospective-pretest design (then-test), recent guidelines suggest adjusting then-test estimates for competing explanations. This prospective longitudinal study investigated recalibration response shift after adjusting for implicit theories of change in patients undergoing spinal surgery. The Oswestry Disability Index (ODI) and Short Form-36 (SF-36) were collected before surgery and at 6 weeks and 3 months after spinal decompression surgery. Then-tests of the measures were also collected at all post-tests. Recalibration response shift was operationalized as the then-minus-pre difference score on the evaluative SF-36. Implicit theories of change were operationalized as the then-minus-pre difference score on the perception-based ODI. Improved vs. No-Effect patient groups were compared using the Minimally Important Difference (+/- 15 points) as a cut-off on the Visual Analogue Scale (VAS) items for back and leg pain. Logistic regression analyses investigated whether recalibration response shift had an independent effect distinguishing patient groups, after adjusting for implicit theories of change. The sample (baseline n = 169, mean age 52, 39 % female) was well-educated, and 1/3 were working. All then-minus-pre difference scores were non-zero at each time point and were stable over time. In the adjusted models distinguishing Improved versus No Effect groups, then-minus-pre ODI difference scores were significant in the majority of the adjusted models at all timepoints, but only one then-minus-pre SF-36 difference score-for physical functioning recalibration-was significant and only at 6-weeks post-surgery. This suggests that implicit theories of change bias the estimation of post-surgical outcomes, but that recalibration response shift biased only the estimation of physical functioning and only at 6 weeks post-surgery. Recalibration response shift and implicit theories of change can both be sources of bias in patient-reported outcome assessment. Our findings suggest that implicit theories of change are a greater threat to validity in this patient sample. Future research using the then-test should control for implicit theories of change to minimize misspecification of effects.