It is frequently the case that it is necessary to be able to detect small changes in patient bone mineral density (BMD) rather than to determine accurately, its absolute value. This occurs when it is necessary to follow the progress of drug therapy. Given that the changes involved may take place over months or years it is essential to be certain that any apparent changes in BMD are real and not instrumental artefacts. In particular, repeated measurements on a particular phantom should give consistent, identical results. Unfortunately, no DXA instrument is perfect and its ability to reliably detect changes in BMD is indicated in terms of what is called its precision [1].