Background. Efficacy of task-oriented training can be reliably trusted only when the inherent measurement variability is determined. The Actual Amount of Use Test (AAUT) and the Motor Activity Log (MAL) have been used together as measures of spontaneous arm use after an intervention; however, the minimal detectable change (MDC) of AAUT and MAL has not been addressed. Objective. To compare the MDC90 of the AAUT and the MAL in the context of a randomized controlled trial of a neurorehabilitation intervention, the Extremity Constraint-Induced Therapy Evaluation trial. Methods. A preplanned secondary analysis was conducted using pre-post test data from the control group. Estimated MDC90 were normalized to the maximum value of the scale of the AAUT and the MAL for each subscale: amount of use (AAUTa, MALa) and quality of movement (AAUTq, MALq). Results. The MDC90 of the AAUTq and the MALq were 14.4% and 15.4%, respectively. However, the MDC90 required greater change for the AAUTa (24.2%) than the MALa (16.8%). The training-induced spontaneous arm use exceeded the MDC90 for the MAL but fell below that for the AAUT immediately after the intervention and at 1-year follow-up visit. Conclusions. The greater variability and insensitivity to treatment effect for the AAUTa is likely because of the low resolution of its scoring system. As such, there is a considerable need to develop valid and reliable tools that capture purposeful arm use outside the laboratory, perhaps through leveraging new sensing technologies with objective activity monitoring.