From analysis menu of SPSS, the crosstabs procedure in descriptive statistics is used to create contigency tables also known as two-way frequency table, corss tabulation, which describe the association between two categories variables.
In a crosstab, the categories of one variable determine the rows of the contingency table, and the categories of the other variable determine the columns. The…
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Chi-square test is a non-parametric test. The assumption of normal distribution in the population is not required for this test. The statistical technique chi-square can be used to find the association (dependencies) between sets of two or more categorical variables by comparing how close the observed frequencies are to the expected frequencies. In other words, a chi square () statistic is used…
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)
