PURPOSE: There is no "gold standard" measure for moderate to vigorous physical activity (MVPA); some error is inherent to self-report and device-based measures. Few studies have examined agreement between self-report and device-based measures in the intervention trial context or whether the difference between measures is influenced by intervention participation. METHODS: MVPA was measured at baseline and after 6 months by Active Australia Survey (AAS) and by the GT1M accelerometer (≥1952 counts per minute) in the intervention (n = 135) and usual care control (n = 141) participants of a randomized trial targeting weight loss by MVPA increases and energy intake reductions in adults with type 2 diabetes. Agreement (for each group at each assessment) was examined using the Bland-Altman approach and regression-based modeling. Because the differences between MVPA measures varied with average values ([AAS + GT1M] / 2), they were examined as a percentage of average physical activity. t-tests were used to assess unadjusted group differences and changes over time. ANCOVA models tested intervention effects on measurement error at follow-up, adjusted for baseline. RESULTS: Agreement worsened, and variability in the difference measures became greater, as the average amount of MVPA increased. Measurement error differed significantly between groups at follow-up (P = 0.010) but not at baseline (P = 0.157) and changed significantly within the intervention group (P = 0.001) but not the control group (P = 0.164). There was a statistically significant effect of the intervention on measurement error (P = 0.026). CONCLUSIONS: Measurement error of self-report relative to the accelerometer appeared to be affected by intervention. Because measurement error cannot be definitively attributed to self-report or accelerometer, it would be prudent to measure both in future studies.

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