An ADL monitoring system uses a set of sensors each adapted to respond to an activity and to generate a sensor output signal representative of the detected activity level or type. An activity density map is formed. The activity level or type is compared with a range of activity levels or types represented in a map which characterized a reference spread of activity levels over the same time period as the activity density map. A probability analysis is then used to identify initial anomaly points. For these the initial anomaly points a test of activity permutations is carried out to find timeslots in the activity density map which may be reordered to remove the initial anomaly points. In this way anomalies at the level of individual timeslots can be identified and the permutation approach makes the system robust to changes in the order in which activities are carried out by a subject.
