[Test graduate-level ability to evaluate validity, reliability, and bias of wearable accelerometers used to measure physical activity in longitudinal clinical and population health research. Assess understanding of construct and criterion validity, test–retest and inter-device reliability, sources of systematic and differential bias (wear-time, placement, algorithms, selection), longitudinal challenges (calibration drift, firmware/algorithm updates, missingness, reactivity), and study design and statistical strategies for mitigation and harmonization.]