BMJ 2009;338:a3167

Douglas G Altman, professor of statistics in medicine1, J Martin Bland, professor of health statistics2 1 Centre for Statistics in Medicine, University of Oxford, Wolfson College Annexe, Oxford OX2 6UD, 2 Department of Health Sciences, University of York, York YO10 5DD

Continuous data arise in most areas of medicine. Familiar clinical examples include blood pressure, ejection fraction, forced expiratory volume in 1 second (FEV1), serum cholesterol, and anthropometric measurements. Methods for analysing continuous data fall into two classes, distinguished by whether or not they make assumptions about the distribution of the data.