Reliability and Validity of the Severity of Illness Index

The authors discuss the objectives and definition of the Severity of Illness Index, which has been developed and refined at The Johns Hopkins University over the past 5 years. In addition, the training program for raters, the method used to ascertain reliability, and data from reliability testing in 18 hospitals are described. After at least 2 months' experience with severity scoring, the average agreement between hospital raters and the staff reliability rater varied between 90.8% and 97.7%, with an overall weighted average agreement of 93.5%. Several methods to evaluate the validity of the Index are presented. The conclusion is that the Severity of Illness Index is a reliable and valid tool for measuring inpatient severity of illness.