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The second example comes from a follow-up study of lung cancer and coronary artery disease in
relation to cigarette smoking:
Mortality rates per 100,000 person-years from lung cancer and
coronary artery disease for smokers and nonsmokers of cigarettes
Smokers Nonsmokers Ratio Difference
Cancer of the lung 48.3 4.5 10.8 44
Coronary artery disease 294.7 169.5 1.7 125
Source: 1964 Surgeon General's Report on Smoking and Health, page 110, quoted in Joseph Fleiss,
Statistical methods for rates and proportions, 2nd edition, page 91
The rate ratio for the relation between smoking and lung cancer mortality is much larger than that
between smoking and coronary artery disease mortality, but the rate difference is much larger for
coronary artery disease mortality. These figures are usually interpreted to mean that lung cancer
mortality is more closely associated with cigarette smoking than is coronary artery disease mortality;
elimination of cigarette smoking would lead to a proportionate reduction in lung cancer mortality
greater than the proportionate reduction in coronary artery disease mortality. However, the
reduction in the number of deaths from lung cancer would be smaller in magnitude than the
reduction in deaths from coronary artery disease. These issues will be explored in detail in the
section Measures of Impact, later in this chapter.
Concept of relative risk
Nevertheless, for the most part we use relative risk as the basic measure of strength of association
between a characteristic and the development of a condition.
General approach - compare observed to “expected”.
Concept of relative risk is operationalized by :
a. Cumulative incidence ratio (CIR), also called risk ratio
b. Incidence density ratio (IDR), also called rate ratio
c. Odds ratio (OR), which estimates CIR and IDR under certain circumstances.
General formula:
Incidence in “exposed” I 1
Inc i de nc e ra ti o = ————————————— = ———
Incidence in “unexposed” I 0
You may recall from the chapter on standardization that the SMR can be thought of as a ratio of
“observed” to “expected” mortality rates. In fact, the concept of observed and expected can be
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www.epidemiolog.net, © Victor J. Schoenbach 2000 7. Relating risk factors to health - 168
rev. 10/9/2000, 5/8/2001, 2/19/2002, 4/26/2002