We would like to thank Horn for her supportive and insightful commentary1 on our article,2 emphasizing the need to pursue our next goal toward conducting comparative effectiveness research3 between the United States and Israel. It is our intention to do so using a practice-based evidence4–6 study design that will include expanded data from both countries on patient characteristics, treatment coding, and functional outcomes.
We agree that differences in health care systems and physical therapy settings between countries, and differences found in risk-adjusted functional outcomes by country, offer a unique opportunity to identify ways to achieve best possible outcomes. We take this opportunity to call for additional health care practitioners and researchers around the world, using a variety of health settings and care, to join us in this exciting endeavor. The more variance we have in the data analyzed, the greater the potential to discover best treatment options for the benefit of our patients.7
Horn raised concern regarding the possibility that in trying to minimize patient selection bias by applying strict patient selection criteria using a minimum number of patients and completion rate per therapist, we might have actually increased patient selection bias due to the consequent and significant reduction in sample size analyzed. We agree with Horn that a valid way to test the generalizability of our findings in relation to patient selection criteria would be to compare our results with those that use all patients with admission and discharge data.
Therefore, we conducted an additional identical analysis of known-groups construct validity using all patients receiving knee rehabilitation who had admission and discharge functional status scores. The sample size increased to 9,584 and 10,092 patients in Israel and the United States, respectively, roughly doubling the sample size for Israel and tripling it for the United States, compared with the original analyses. Using these less strict inclusion criteria, completion rates decreased from 60% to 46% for the Israeli sample and from 63% to 33% for the US sample. Interestingly, compared with the original known-groups construct validity analyses described in our article,2 we found exactly the same interaction terms between the independent variables tested and country to be significant. Specifically, P values of interaction terms for sex, age groups, symptom acuity, surgical history, exercise history, and medication use at intake for the expanded sample (compared with the original sample, as presented in Tab. 32) were .563 (.264), <.001 (<.001), .139 (.137), <.001 (.002), .235 (.132), and <.001 (.001), respectively. Graphic comparisons of adjusted discharge functional status scores between countries for all independent variables tested were practically identical to those presented in Figure 2.2
These additional analyses and results suggest that the strict patient selection criteria used in our study did not affect patient selection bias in relation to the known-groups validity tested using the techniques described. Furthermore, these results support Horn's commentary that concerns raised about the “huge” potential for patient selection bias in observational studies, in some cases, might be overestimated.
- © 2011 American Physical Therapy Association
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