Contingency tables amp significance tests for categorical variables Aya Elwazir Teaching assistant of medical genetics FOMSCU PHD student University of Sheffield Case No Treatment Troponin ID: 1038091
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1. Statistics for biological dataContingency tables & significance tests for categorical variablesAya ElwazirTeaching assistant of medical genetics, FOMSCUPHD student, University of Sheffield
2. Case No.TreatmentTroponinAbdominal problems1Aspirin0.032Yes2Aspirin0.045Yes3Aspirin0.028No4Placebo0.018Yes5Placebo0.030No6Placebo0.021NoCategoricalContinuousCategoricalIs daily aspirin use associated with higher frequency of abdominal problems? H0Daily aspirin use is associated with lower OR not associated with frequency of abdominal problems A look at the data!H1Daily aspirin use is associated with higher frequency of abdominal problems
3. Two by two tableAbdominal problemsYesNoTreatmentAspirinPlaceboFrequency distribution of two categorical variables simultaneouslyAllows us to assess if there is a relationship between the two variablesIs daily aspirin use associated with higher frequency of abdominal problems?
4. Two by two tableAbdominal problemsYesNoTreatmentAspirin2 1 Placebo1 2 Count‘frequency’Case No.TreatmentAbdominal problems1AspirinYes2AspirinYes3AspirinNo4PlaceboYes5PlaceboNo6PlaceboNo
5. Two by two tableAbdominal problemsYesNoTreatmentAspirin2 1 Placebo1 2 Total33Total336Total Proportion/percentage
6. Two by two tableAbdominal problemsYesNoTreatmentAspirin2 (0.67)1 (0.33)Placebo1 (0.33)2 (0.67)Total33Total336Total Proportion/percentage
7. Two by two tableAbdominal problemsYesNoTreatmentAspirin2 (67%)1 (33%)Placebo1 (33%)2 (67%)Total33Total336Total Proportion/percentage
8. Two by two tableAbdominal problemsYesNoTreatmentAspirin2 (67%)1 (33%)Placebo1 (33%)2 (67%)P value?Is daily aspirin use associated with higher frequency of abdominal problems?
9. Hypothesis Testing for categorical variablesChi square testFisher Exact test
10. Chi square testCompares distribution of two categorical variables in a contingency table to see if they are relatedMeasures difference between what is actually observed in the data and what would be expected if there was truly no relationship between the variables Overall totalRow total x Column totalE=
11. Chi square testAbdominal problemsYesNoTreatmentAspirin33.326.7Placebo31.725.3ObservedExpected Overall totalR total x C totalE=Abdominal problemsTotalYesNoTreatmentAspirin431760Placebo223557Total6552117
12. Abdominal problemsTotalYesNoTreatmentAspirin431760Placebo223557Total6552117Chi square testObserved Overall totalR total x C totalE=X2= 12.95P= 0.00032H0Daily aspirin use is associated with lower OR not associated with frequency of abdominal problems Accept or reject?
13. Fisher Exact testUsed instead of Chi square when >20% of cells have expected values < 5 Or any cell has a count< 1Abdominal problemsYesNoTreatmentAspirin4.316.7Placebo11.73.6Expected
14. Case No.TreatmentTroponinAbdominal problems1Aspirin0.032Yes2Aspirin0.045Yes3Aspirin0.028No4Placebo0.018Yes5Placebo0.030No6Placebo0.021NoCategoricalContinuousCategoricalIs daily aspirin use associated with lower troponin levels?A look at the data again!Significance tests for continuous variables
15. Statistics for biological dataCourse ObjectivesIntroduction to statistics1. Contingency tables & testing for categorial variables2. Normality testing & Descriptive statistics3. Testing for continuous variablesLots of practice!