2x2 Tables & Calculating Ratios
A Few Additional Examples
The Scenario:
On a college campus, an outbreak of influenza occurs. Of the 60 students who were vaccinated, 20 students came down with the flu. Of the 90 students who were not vaccinated, 30 of the students were vaccinated.
How to Construct the Table
To set the table up correctly, we need for the students in cell A to be those who had both the exposure and the disease. The exposure in this scenario is being vaccinated. The students in cell A will be students who were vaccinated and came down with influenza.
How to Calculate the Ratios - Relative Risk
To calculate relative risk, we use the incidence rate in the exposed group divided by the incidence rate for the unexposed group.
Incidence Rate for the exposed group is 20/60 = .33
Incidence Rate for the unexposed group is 30/90 = .33
Relative Risk = .33/.33 = 1
A relative risk ratio close to 1 means there is not a association between exposure and disease so student who were vaccinated and those who were not were equally as likely to get the disease.
Odds Ratio
To calculate odds ratio, take cells A x D divided by B x C.
For this scenario, the odds ratio is (20 x 60) / (40 x 30) = 120/120 = 1
The odds of developing influenza were equal for both the vaccinated students and the non-vaccinated students.
The Assignment
The assignment for this will be released in a news item. You are to complete the Worksheet provided. For the two tables on the worksheet, you are to calculate the relative risk ratios and odds ratio.