Rate Ratio

When the risk is expressed as a rate (since incidence is a rate), the corresponding term for risk ratio is Rate Ratio. The major difference is that the Rate Ratio employs incidence rate (in person-time) instead of cumulative incidence. However, the formula remains unchanged:

Rate Ratio = CI_e/ CI_u

Here, CI_e represents the incidence rate among exposed and CI_u represents the incidence rate among unexposed.

The 2×2 contingency table would be as under:

Outdoor air pollution?	No. with Respiratory illness	Person-years of disease-free follow-up	Incidence Rate per 1000 person-years
Yes	400	6400	400/6400 = 62.5
No	100	8400	100/8400 = 11.9

The Rate Ratio would be 62.5/11.9 = 5.25

Interpretation: Those living in city A had 5.25 times the rate of respiratory illness compared to those living in city B.

Often, Rate Ratios are interpreted as if they were Risk Ratios, but it is more appropriate to interpret them as ratios of rates rather than risk.