Signifigance of 2 X 2 Contingency Tables, Relative Risk, Odds Ratio
The significance and usefulness of 2 X 2 contingency tables:
Considering the 2X2 table: A=number of exposed w/ disease; B=number of exposed w/out disease; C=number not exposed but w/ disease; D=number w/out disease or exposure.).
Disease
Yes No
Exposed Yes A B
No C D
The significance of an odds-ratio =, <, or > 1:
An OR=1 implies that the odds of exposure are equal among the cases and controls and suggests that a particular exposure is NOT a risk factor for the disease being studied. An OR<1 implies that the factor has a protective effect (i.e., reduces the risk of the disease or health outcome). An OR>1 indicates that the exposure is associated with an increased risk for the disease or health outcome.
The odds ratio is the ratio such that (probability that a case was exposed): (probability that a control was exposed)
Recall that the odds ratio is associated with case-control studies or cross-sectional studies. A simplified formula to calculate the OR is AD/BC.
2 X 2 Table Sample Problems:
Using a 2x2 Table to calculate Odds Ratio (OR)
Using the following sample 2 X 2 table, calculate the Odds Ratio and discuss the significance of the results.
Disease
Yes No
Exposed Yes 74 44
No 124 54
Answer:
OR = AD/BC = (74 x 54)/(44 x 124) = (3,996 / 5,456) = 0.73
Because the OR<1, it implies that the “exposure” factor has a protective effect (i.e., reduces the risk of the disease or health outcome). The “exposure” could be referring to “exposure” to a vaccine.
Using a 2 X 2 Table to calculate Relative Risk (RR)
Using the following 2 X 2 Table to calculate the Relative Risk (RR) and discuss the significance of the results.
Disease
Yes No Row Totals
Exposed Yes 189 849 A+B = 189+849 = 1038
No 24 1249 C+D = 24+1249 = 1273
Answer:
RR = (Incidence rate in the exposed)/(Incidence rate in the non-exposed)
