Apparatus for detecting non-compliance patterns on a series of used blister sheets previously returned by the same patient, reads a code (2) on each blister sheet (1) providing, inter alia, information relating to the position of each blister on the blister sheet. This information is passed by the code reader (4) through a key (15) to an image (5) of the blister sheet appearing on one-half (14) of a split screen (6). The screen (14) displays a picture of a blister sheet with its blister positions marked by dots or rings (8). By means of the key (15), which may take the form of a joystick, an image of a disc (9) can be placed over each of the blister positions where an unopened blisters occurs. The image of the blister sheet together with the discs is then transferred by operation of a key (10) to the second half (11) of the split screen (6) screen where it is superimposed on a slightly offset stack (13) of used blister sheets previously returned by the same patient. The positions of the discs (9) are compared by the apparatus with corresponding positions on previously returned blister sheets and, if a coincidence is detected, the apparatus produces a change in the appearance of the corresponding disc on the blister sheet on the top of the stack. A change in a disc appearance suggests that there may be a pattern of non-compliance that can then be investigated further.