1. A display arrangement, comprising: an image display device to display at least one image representing advertising material for goods or services for sale in a shop, the image display device including a first camera directed towards locations adopted by viewers viewing the image display device, and the first camera being configured to capture images of faces of the viewers as the viewers view the image display device; one or more further cameras directed towards other respective locations, at least one of the further cameras being directed towards a checkout of the shop to capture images of purchasers; a face detector for detecting human faces of the purchasers and the viewers in images captured by the cameras, the face detector being arranged to detect a total dwell time of each of the faces detected by the first camera, and the dwell time is a time that each of the faces is directed towards the image display device; and a correlation detector to correlate faces which appear in images captured by both the first camera and at least one of the further cameras by detecting whether a purchaser whose face is detected at the checkout was also detected as a viewer of the image display device.