Standardized Infection Ratio (SIR) Statistical Method (SIR)

This indirect standardization method accounts for differences in the risk of surgical site infections among a group of procedures. A SIR is the number of observed infections divided by the number of expected infections. The expected number is based on the national average, the number of procedures performed by a hospital, and historical data for those procedures. A SIR of 1.0 means the observed number of infections is equal to the number of expected infections. • A SIR above 1.0 means that the infection rate is higher than that found in the "standard population." For HAI reports, the standard population comes from data reported by the hundreds of U.S. hospitals that use the NHSN system. The difference above 1.0 is the percentage by which the infection rate exceeds that of the standard population. • A SIR below 1.0 means the infection rate is lower than that of the standard population. The difference below 1.0 is the percentage by which the infection rate is lower than that experienced by the standard population. The number of expected infections is calculated by multiplying the number of procedures (at each hospital) by the NHSN Pooled Mean Rate. The result is divided by 100 (to remove the percent). The SIR is found by dividing the sum of the observed number of SSIs by the sum of the expected number of SSIs across the different procedures.