Rate Ratio
- A rate is an expression of the frequency with which an event occurs in a defined population in a specified period of time.
- The numerator of a rate is the number of events (e.g., disease occurrences), while the denominator is usually person-time.
- Using the 2×2 table (Please see Aschengrau & Seage, Chapter 3):
| Disease + | Disease - | Person-Time (PT) | |
| Exposure + | a | -- | PT exposed |
| Exposure - | c | -- | PT unexposed |
| Total | a+c | -- | Total PT |
Rate of Disease in the Exposed: a/Person-Time exposed
Rate of Disease in the Unexposed: c/Person-Time unexposed
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The rate ratio is the ratio of two rates, usually exposed/unexposed:
Rate Ratio: [a/Person-time exposed]/[c/Person-time unexposed] - Interpretation: During time t, the rate of developing disease was (n times higher/n times less likely) in exposed persons compared with non-exposed persons in this population per person-time of observation.
