Statistical basis and methods

All consecutive admissions (patients aged 18 years or older) to adult ICUs in the participating hospitals were eligible for enrolment, but burn patients, coronary care patients, and cardiac surgery patients were excluded from the statistical analyses. Interoperator quality was controlled by having each site co-ordinator complete a second set of forms for a 5 per cent random sample of the patients studied at that ICU. Kappa statistics ( Fleiss 1981) and intraclass correlation coefficients ( Fleiss...

1986) were used to assess the quality of the data. Data collection included patient demographic information, all variables necessary for computing the original SAPS, a set of new variables that might be included in SAPS II, and vital status at hospital discharge. The physiological variables were recorded by the data collectors as the worst value in the first 24-h period in the ICU. The worst value was defined as the value that would have been assigned the greatest number of SAPS points in the original SAPS score.

To develop SAPS II, 65 per cent of the available patients were randomly selected to form the developmental dataset, while the remaining 35 per cent became the validation dataset. Each of the possible explanatory variables was independently evaluated for its association with hospital mortality. These bivariate analyses were used to screen through the set of independent variables to identify a smaller subset associated with hospital mortality.

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