Specialty › Tests on summarized data

Compare two incidence rates

Comparison of two incidence rates from typed-in event counts + person-time per group. Wald log-rate CI for the rate ratio + score-based equality test.

What is Compare two incidence rates?

For two arms, the rate ratio RR = λ₁ / λ₂ has approximately normal log on the scale log(RR), with SE = √(1/e₁ + 1/e₂). The Wald CI is exp(log RR ± z·SE), which is asymmetric on the original scale (right by design).

The score test for H₀: RR = 1 follows Sahai & Khurshid (1996) and is more robust than the Wald test when one event count is small.

When should I use Compare two incidence rates?

Two-arm follow-up study with rate outcomes (e.g. infections / patient-years).

What data does it need?

Event count + person-time for each of two groups.

What does it report?

Per-group rate + rate ratio with CI + rate difference + z + p for equality.

How do I interpret the result?

RR is the standard reporting metric for rate ratios; RD is for absolute risk differences. Report both when each is informative.