This is a very good question because we simply do not know where to draw the cut off lines. It depends on what is thought to be an accepted cause and effect end point. From the point of view of diabetes we do not know the answer but as regards musculoskeletal disorder it is believed that an optimal level of 25-OH vitamin D is of the order of 50 - 75 nmol/l.

If the level of 50 - 75 nmol/l is accepted as the normal range then a population based study can be conducted in order to established the number of people who are deficient. These results can be compared in time over the seasons, in years over the decades and in different populations