Block 1: Epidemiology
EPI4-5 THE ODDS RATIO

SESSION OBJECTIVE

At the end of this session you should have an understanding of the relationship between the OR and the IRR

LEARNING ACTIVITIES:

The relationship between the Odds Ratio, OR, and the Incidence Rate Ratio, IRR, is a little more difficult to grasp.

  D+ D-  
E+ a c M1
E- b d M0
  N1 N0 T

From the table above it can be seen that

IRR = I1/I0 = (a/PYR1)/(b/PYR0)

where:
   a is the number of disease cases in the exposed group
   b is the number of disease cases in the non-exposed group
   PYR1 is the person-years in the exposed group
   PYR0 is the person-years in the unexposed group

If the controls are representative of the exposure distribution in the population that generates the cases, then c and d should be good estimators of PYR1 and PYR0.

And so, IRR ~ ad/bc


Interactive Examples

The Examples listed below have been included in order to illustrate the concepts discussed in this Section.
Question 1 Question 3 Question 5
Question 2 Question 4 Question 6




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General Introduction to Occupational Health: Occupational Hygiene, Epidemiology & Biostatistics by Prof Jonny Myers is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.5 South Africa License
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