[Beyond normal values--practical experiences with the standard deviation score (SDS) for the assessment of lung function measurements in acute bronchial asthma]

Expressing results of pulmonary function tests as a percentage of the subject's predicted value (% predicted) has been widely adopted. This method has some serious disadvantages: The range of values % predicted in the reference population depends on the body height and varies from index to index. An alternative is to use the standard deviation score (SDS), which is derived by dividing the difference between the recorded and predicted value by the standard deviation of the residuals (SDS = (recorded-predicted)/SD).