To this end a literature review was a good starting point, but there is no substitute for the detailedhere was an intensive period of familiarisation. Familiarisation provided the necessary context for research and gave an appreciation of the concerns and sensitivities of the staff involved. This immersion into the life of the unit was also essential in order to gain a view of what would be required of any new analytical methods if they were to be useful. Cardiac surgeons are very highly skilled but not, by and large, in mathematical fields. Although classical statistical methods are applicable to the overall analysis of mortality data, in the present context, they often obscure rather more than they illuminate.
Complicated analysis leads to poor understanding and thus mistrust of the methods being used.
Once familiarisation was underway, the first analytical objective was to devise an alternative method of estimating the risks of perioperative mortality that reflected a contemporary British surgical practice and so improve estimation accuracy. This method was constructed and tested using 4,318 case records. The data was fully computerised, had been validated and was maintained to a very high standard by a clinician within the surgery unit. In surgical terminology, 'isolated' is used to indicate that only one procedure is carried out during the operation to distinguish from cases where, say, a bypass and a valve replacement are performed. The remaining cases were evenly split between isolated valve operations, and all other procedures.