An appraisal of the SD(IR) as an estimate of true individual differences in training responsiveness in parallel‐arm exercise randomized controlled trials

Calculating the standard deviation of individual responses (SD(IR)) is recommended for estimating the magnitude of individual differences in training responsiveness in parallel‐arm exercise randomized controlled trials (RCTs). The purpose of this review article is to discuss potential limitations of parallel‐arm exercise RCTs that may confound/complicate the interpretation of the SD(IR). To provide context for this discussion, we define the sources of variation that contribute to variability in the observed responses to exercise training and review the assumptions that underlie the interpretation of SD(IR) as a reflection of true individual differences in training responsiveness. This review also contains two novel analyses: (1) we demonstrate differences in variability in changes in diet and physical activity habits across an intervention period in both exercise and control groups, and (2) we examined participant dropout data from six RCTs and found that significantly (P < 0.001) more participants in control groups (12.8%) dropped out due to dissatisfaction with group assignment compared to exercise groups (3.4%). These novel analyses raise the possibility that the magnitude of within‐subject variability may not be equal between exercise and control groups. Overall, this review highlights that potential limitations of parallel‐arm exercise RCTs can violate the underlying assumptions of the SD(IR) and suggests that these limitations should be considered when interpreting the SD(IR) as an estimate of true individual differences in training responsiveness.