Rate Ratio

  1. A rate is an expression of the frequency with which an event occurs in a defined population in a specified period of time.
  2. The numerator of a rate is the number of events (e.g., disease occurrences), while the denominator is usually person-time.
  3. 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

  1. The rate ratio is the ratio of two rates, usually exposed/unexposed:
    Rate Ratio: [a/Person-time exposed]/[c/Person-time unexposed]
  2. 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.