In the SIR calculation, what does the 'observed' represent?

• A. The Predicted Number of Infections
• B. The Total Number of Patients Admitted
• C. The Actual Number of Infections Observed
• D. The Rate per 1,000 Patient-Days

Answer: (C)

In SIR calculations, the observed value is the actual count of infections that were recorded in the study population during the specified time period. It’s the real number of events you counted, and it serves as the numerator in the ratio that compares what happened to what would be expected based on a reference population. The expected number is obtained by applying standard incidence rates to the study population’s person-time, and then you compare observed to expected (observed divided by expected) to see if infection risk is higher or lower than expected.

So the observed isn’t a predicted number, nor a total of admissions, nor a rate per 1,000 patient-days. It’s simply the true count of infections that occurred. For example, if you observed 8 infections in the cohort but expected 4 based on reference rates, the SIR would be 2.0, indicating twice the expected incidence.