Intended for healthcare professionals

Rapid response to:

Practice Statistics Notes

The cost of dichotomising continuous variables

BMJ 2006; 332 doi: (Published 04 May 2006) Cite this as: BMJ 2006;332:1080

Rapid Response:

Discarding useful evidence.

Altman and Royston have done a great service in discouraging the
categorisation of continuous data in health research. A particularly bad
case of this is the tendency of researchers (and indeed public statistical
agencies) to use often broad age categories for data that originally had
single year of age. In addition to the serious statistical disadvantages
of this practice, there has come to be a habit of believing that certain
age cut-off points matter. The evidence is clear that being over 65 has
little relevance in terms of disease or health care costs, and the
patterns found are of continuous twice differentiable curves. Perhaps we
should encourage new terminology, such as 'evidence destruction', in place
of the more neutral 'categorisation'.

Competing interests:
None declared

Competing interests: No competing interests

09 May 2006
Charles Normand
Edward Kennedy Professor of Health Policy and Management
Trinity College Dublin, Dublin 2