Bradford combines measures of both frequency and duration of absence, with a greater weighting placed on frequency. The formula S2D, or S x S x D, is used to calculate a 'score'or 'index'for a given period, usually a rolling year, where S is the number of spells of absence, and D is the aggregate number of days absent.

This results in employees with the same amount of sick leave but different numbers of absences having widely disparate 'scores'. For example, three people absent for 25 days would have scores of 25,225 and 900 if their absences were spread over one, three and six absences.

Typically, an employer will set a 'threshold score' beyond which the personnel department or occupational health department will become involved.

But the system seems not to be the work of Bradford University. Dr Alistair Ostell, a senior lecturer in its management centre, says: 'We have not been able to identify who originated this work. '