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Statistical Measures

Relative Risk & Odds Ratio

Relative Risk (RR)

Table 5
Number of Cases of Disease X by Sex, Smallville, 2004

 
Disease X
  
  Yes No Total
Female a    46 b    1,438 1,484
Male c    18 d    1,401 1,419

The term “two-by-two” refers to the two variables (sex and disease status), each with two categories. The outcome (illness or not) is shown at the top of the table and exposer or risk factor is shown along the left side. Note the letters assigned to each cell of the table (a-d). They are important in calculating the risk in each group.

Example
Using the data in the table above, we can calculate the relative risk of Disease X for females vs. males. First, we must calculate the risk of illness among females and among males:

To calculate the RR for females vs. males, females are considered the group of primary interest and males are the comparison group. The formula is:

So we can say that the risk of Disease X in females appears to be 2.4 times higher than the risk in males.