A method of detecting periodic activity patterns associated with the viewing of audio video content is described. The method includes: recording activity data in an activity log; suppressing the activity log one or more times to suppress non-relevant activity data thereby producing one or more sets of suppressed activity data; passing the one or more sets of suppressed activity data through a signal processing function to convert the one or more sets of suppressed activity data to one or more frequency responses; and analysing the one or more frequency responses to detect the periodic activity patterns. Related apparatus and methods are also described.