Measures of Comparison

1. Comparison Measures slide #1 Measures of Comparison 

2. Comparison Measures slide #2 Purpose Summarize relationship between exposure and disease by comparing at least two measures of disease frequency Overall rate of disease in an exposed group says nothing about whether exposure is a risk factor for or causes a disease. This can only be evaluated by comparing disease occurrence in an exposed group to another group that is usually not exposed. The latter group is usually called the comparison or reference group. Comparison is the essence of epidemiology.

3. Comparison Measures slide #3 Two Main Options for Comparison 1. Calculate ratio of two measures of disease frequency ( a measure in exposed group and a measure in unexposed comparison group) 2. Calculate difference between two measures of disease frequency (a measure in exposed group and a measure in unexposed comparison group)

4. Comparison Measures slide #4 Data Set Up: Two by Two Table

5. Comparison Measures slide #5 For Incidence Rates

6. Comparison Measures slide #6 Rate/Risk Ratio (also called Relative Risk) Comparing disease occurrence among exposed with disease occurrence among comparison group (usually unexposed) in a ratio measure. 

7. Comparison Measures slide #7 Rate/Risk Ratio (also called Relative Risk) RR= Rate or risk in exposed group (Rexp) / Rate or risk in unexposed group (Runexp) For CI: CIexp / CIunexp = a / (a+b) / c / (c+d) For IR: IRexp / IRunexp = a / PTexp / c / PTunexp

8. Comparison Measures slide #8 Rate/Risk Ratio (also called Relative Risk) Purpose: Gives information on the relative effect of the exposure on the disease. Tells you how many times higher or lower the disease risk is among the exposed as compared to the unexposed. Is commonly used in etiologic research

9. Comparison Measures slide #9 Rate/Risk Ratio RR=1.0 -- no association between exposure and disease RR=2.0 -- two times the risk of disease in the exposed compared to the unexposed

10. Comparison Measures slide #10 Rate/Risk Ratio (cont’d) RR=1.6 -- 1.6 times the risk of disease in the exposed compared to the unexposed or 60% increased risk of disease in the exposed (1.6 - 1.0 = .60 = 60%) RR = 0.5 -- 0.5 times or ½ the risk of disease in exposed compared to unexposed.

11. Comparison Measures slide #11 Example: Cohort study of hypertension and cardiovascular morbidity and mortality(Nurses Health Study)

12. Comparison Measures slide #12 Example: Cohort study of hypertension and cardiovascular morbidity and mortality (Nurses Health Study)

13. Comparison Measures slide #13 Example of an R x C Table in a Study of Magnetic Field Exposure and Leukemia

14. Comparison Measures slide #14 Difference Measures Comparing disease occurrence among the exposed with the disease occurrence among the unexposed comparison group by subtracting one from the other.

15. Comparison Measures slide #15 Risk/Rate Difference (also called Attributable Risk/Rate) RD=Rate or risk in exposed (Rexp)– Rate or risk in unexposed (Runexp) For CI: CIexp - CIunexp = a / (a+b) - c / (c+d) For IR: IRexp - IRunexp = a / PTexp - c / PTunexp RD = 0 when there is no association

16. Comparison Measures slide #16 Risk/Rate Difference (continued) Purpose: Gives information on the absolute effect of exposure on disease occurrence. the excess disease risk in the exposed group compared to the unexposed group. the public health impact of an exposure, that is, how much disease would be prevented if the exposure were removed. This assumes that the exposure causes the disease.

17. Comparison Measures slide #17

18. Comparison Measures slide #18 Comparison of RR and RD

19. Comparison Measures slide #19 Population Risk/Rate Difference (PRD) Purpose: Measures excess disease occurrence among the total population that is associated with the exposure. Helps to evaluate which exposures are most relevant to the health of a target population.

20. Comparison Measures slide #20 Population Risk/Rate Difference (PRD) Two formulas for PRD: PRD = (RD) (Pexp) where Pexp = proportion of population that is exposed, and RD is the risk or rate difference PRD = Rtotal - Runexp where Rtotal = risk/rate in total population and Runexp = risk/rate among unexposed

21. Comparison Measures slide #21 Population Risk/Rate Difference

22. Comparison Measures slide #22 Population Risk/Rate Difference Note the dependence of PRD on prevalence of exposure. What would the excess of non-fatal heart attack due to hypertension be if the prevalence of hypertension were 1% rather than 11.2%? A relatively weak risk factor (in terms of relative risk) that is quite prevalent could account for more of disease incidence in a population than a stronger risk factor that is rarely present.

23. Comparison Measures slide #23 Calculating Measures of Comparison for Cigarette Smoking and Lung Cancer* Simple Rates Death rate from lung cancer in smokers 0.96 / 1,000 / year Death rate from lung cancer in non-smokers: 0.07 / 1,000 / year Prevalence of smoking in population: 56% * Estimated data from Doll and Hill. Br J Med 1:1399-1410, 1964.

24. Comparison Measures slide #24 Calculating Measures of Comparison for Cigarette Smoking and Lung Cancer* Compared Rates Rate Ratio: 0.96 / 1,000 / year / 0.07 / 1,000 / year = 13.7 Rate Difference: 0.96 / 1,000 / year – 0.07 / 1,000 / year = 0.89 / 1,000 / year Population Rate Difference: 0.89 / 1,000 / year x 0.56 =0.50 / 1,000 / year * Estimated data from Doll and Hill. Br J Med 1:1399-1410, 1964.

25. Comparison Measures slide #25 Exercise to Practice Measures of Comparison The 58th annual convention of the American Legion was held in Philadelphia from July 21 until July 24, 1976. People at the convention included American Legion delegates, their families, and other legionnaires who were not official delegates. Between July 20th and August 30th, some of those who were or had been present became ill with a type of pneumonia subsequently named Legionnaire's Disease. No one attending the convention developed the disease after august 30th. Following are the numbers of delegates and non-delegates who developed Legionnaire's Disease during the period July 20 to August 30 (41 day period).

26. Comparison Measures slide #26 Exercise to Practice Measures of Comparison

27. Comparison Measures slide #27 Exercise to Practice Measures of Comparison 1. Compute the "rate" of Legionnaires' Disease among the delegates and non-delegates. What type of measure of disease frequency is this "rate"?

28. Comparison Measures slide #28 Exercise to Practice Measures of Comparison 2. Calculate the "rate" ratio of Legionnaires' Disease among delegates compared to non-delegates. State in words the meaning of this “rate” ratio.

29. Comparison Measures slide #29 Exercise to Practice Measures of Comparison 3. Calculate the "rate" difference of Legionnaires' Disease for delegates. State in words the meaning of this”rate” difference.

30. Comparison Measures slide #30 Further Analysis of Convention Delegates

31. Comparison Measures slide #31 Epidemiology In the News

32. Comparison Measures slide #32 Epidemiology In the News What is the exposure under study? How is it defined? What are the diseases under study? Find the measures of disease frequency. Find the measures of association. What did the investigators do to ensure that the comparisons were “fair?”