There is provided an apparatus (100) for measuring movement in a subject. The apparatus (100) comprises a processor (102). The processor (102) is configured to acquire a ballistography, BSG, signal obtained from the subject by a BSG sensor (104). The acquired BSG signal is indicative of movement in the subject. The processor (102) is configured to analyze the acquired BSG signal to identify an index that quantifies a stability of the acquired BSG signal and determine a measure of a quantity of movement in the subject based on the identified index.