Comparing Two Person-Time Rates (PersonTime2)

### Kevin M. Sullivan, PhD, MPH, MHA, Associate Professor, Department of Epidemiology, Rollins School of Public Health, Emory University, Atlanta, Georgia, USA

## Formulae and Example for a Single Table with Person-time Denominator

For a single person-time table comparing exposed and nonexposed, the notation is depicted in Table 15-10. Estimates of the incidence rate ratio (IRR), the incidence rate difference (IRD), their confidence intervals, and test for interaction are presented. Approximate confidence interval methods are provided but it is important to note that there are better confidence interval methods when the data are sparse.

Table 15-10. Notation for comparing exposed and nonexposed with person-time denominators.

| Exposed | Nonexposed |
Cases | a | b | m1
Person-time | n1 | n0 | n

Estimated incidence rate in the exposed =