Letter to the editor.

Dear Professor Johnson, As representatives of one of the manufacturers whose products were investigated, we would like to comment on the paper of J.H. Campbell, et al., “OASIS I: Retrospective analysis of four different microprocessor knee types”, published in the Journal of Rehabilitation and Assistive Technology Engineering 2020, Volume 7, pages 1–10. We commend the authors for their impressive study that is a valuable contribution to the body of evidence as it confirms the generalizability of smaller studies with microprocessorcontrolled knees (MPK) to real-life conditions in a large sample and assessed injurious falls that may serve as a solid foundation for future costeffectiveness analyses. We congratulate the authors on the big achievement to have completed a study with 602 participants successfully. Nevertheless, we believe that some points in the paper require clarification. First, the authors refer to their study as “retrospective” as the most important aspect of the study design. However, we think it would be more important to note that the study design was crosssectional, which has considerable consequences for the interpretation of data and foundation of conclusions. By nature, cross-sectional study designs assess outcomes at only one point in time and, thus, do not allow for any conclusions on causal relationships to explain the results. Therefore, all the interpretations and conclusions offered by the authors should have been clearly marked as hypotheses that require further study to confirm or reject rather than facts supported by the results. Therefore, it would have been appropriate to also discuss other possible explanations for the results, especially in lieu of the claimed “absence of bias”. Second, as there is no information on the baseline status of the patients included, the study was unable to control for a potential clinician bias in the selection of the different MPK. If clinicians preferred a certain MPK for patients with increased risk of falling and lower baseline mobility, the “parity” of outcomes would conceal a clear difference in effects of the four MPK on injurious falls and mobility. This limitation is important as there is comparative evidence that has already demonstrated that there are meaningful technical and functional differences between the MPK (e.g. Bellmann et al., 2010; Thiele et al., 2014; Thiele et al., 2019; Bellmann et al., 2019). Thus, another hypothesis to explain the results could have been that the Hanger prosthetists apparently choose MPK wisely when fitting their patients, making sure that the final outcomes of patients with different levels of baseline mobility are not significantly different. Third, we find the use of the term “parity” confusing and scientifically inadequate. “Parity” is more of a legal rather than a scientific term. The established statistical approaches to compare the differential effects and outcomes of similar interventions are equivalence and non-inferiority. However, these would have required the use of completely different statistical methods as the simple absence of statistical differences does neither establish equivalence nor non-inferiority. Fourth, another scientifically inappropriate wording affects the interpretation of outcomes of patients in the various age bins. The use of the term “decline” to characterize the differences in outcomes suggests a time series that reflects a decrease in outcomes over time when patients age with their MPK. However, this is not what this study measured. Given that the study was cross-sectional and, thus, assessed outcomes only once the study did not find a decline over time but the unsurprising fact that patients who undergo an amputation later in life start, on average/median, with a lower baseline mobility than younger amputees.