Phil Hurvitz
phurvitz@u.washington.edu
EPI 511, Kukull
Autumn, 2004
October 26, 2004
Assignment 4: Measures of excess risk
1.
You are the epidemiologist hired as a consultant to a pasta making factory. Because of some suspicion on the part of local physicians,
you conduct a study of the occurrence of respiratory disease among the workers at the plant. The analysis generates the following
age-adjusted results: Incidence of respiratory disease in workers exposed to pasta = 200 per 10^5 p-yrs Incidence of respiratory
disease in workers not exposed to pasta = 20 per 10^5 p-yrs Incidence of respiratory disease in the general population = 21 per 10^5
p-yrs On the basis of the above information, answer the following questions:
a.
What is the relative risk (RR) of respiratory disease associated with pasta exposure?
RR = incidence in exposed / incidence in nonexposed =
200 / 20 = 10
b.
Calculate the attributable risk (AR) and population attributable risk percent (PAR%).
AR for exposed = incidence in exposed – incidence in nonexposed =
200/10^5 py – 20/10^5 py = 180/10^5 py
AR in total population = incidence in total population – incidence in nonexposed =
21/10^5 py – 20/10^5 py = 1/10^5 py
PAR% = (inc. in total population – inc. in nonexposed)/ inc. in total population
1/21 * 100% = 4.76%
2.
How can you account for the low PAR% in view of the high AR and RR?
PAR% is low for the same reason AR in the total population is low; the incidence of respiratory
disease is similar for the general population as nonexposed factory workers. AR for exposed is
very high, but this value is not used in calculating PAR% for the total population.
3.
Based on the observed RR and the other available scientific information, you conclude that the association between occupational
pasta exposure and the incidence of respiratory disease is a causal one.
a. What measure answers the following question: “If an exposed worker develops respiratory disease, what is the likelihood
that the disease occurred as a result of his/her employment?
The measure is proportion or percentage of attributable risk for the exposed group.
b.
Calculate the measure in part a.
PAR% for exposed group = (inc. in exposed – inc. in nonexposed)/ inc. in exposed * 100% =
(200 – 20)/200 * 100% = 90%
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This means 90% of the morbidity from respiratory disease among exposed is attributable to
exposure.
4.
The following data are excerpted for the article “Trends in mortality from coronary heart disease and stroke among U.S. Veterans,
1954-1979.” (J Clin Epidemiol 1989;42:245-256). For each age group, calculate the relative risk of death due to stroke associated
with cigarette smoking in these data.
RR = incidence in exposed / incidence in nonexposed:
e.g., for 55-64, RR = (173 / 249963 PY) / (489 / 465066 PY) = 1.52
Smoker Age # of stroke deaths PY at risk inc/1000 PY RR AR (/1000 PY) PAR%
N
55-64
173 249,963
0.69
N
65-74
999 409,387
2.44
N
75-84
2,258 271,558
8.31
Y
55-64
489 465,066
1.05 1.52
0.36 34.18
Y
65-74
1,836 560,320
3.28 1.34
0.84 25.53
Y
75-84
2,588 281,129
9.21 1.11
0.89 9.68
5.
Assume that smoking is causally associated with stroke mortality at all ages, and the data in the table represent the experience of the
entire population of 55 to 84 year old U.S. Veterans during the study period. What is the appropriate measure to answer the following
question:
a. a. In which age group(s) would the elimination of smoking have prevented the greatest proportion of stroke deaths among
veterans in that age group? Why is this measure the most appropriate?
The most appropriate measure for assessing this risk is the proportion or percentage of
attributable risk for the exposed group. This measure indicates the percent of mortality due to
stroke that could be reduced in smokers (rather than in the total population) by cessation of
smoking.
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