8: Data Interpretation and Confounding Exercise
Table 1 below describes data
from a prospective cohort study examining the association between type of work
schedule and the risk of prostate cancer in men.
Table 1: Incidence of
prostate cancer in 14,052 male workers according to type of work schedule
No. of subjects with no
diagnosis of prostate cancer at the time of enrollment in the study
No. of person-years of follow-up
No. of new prostate cancer
Express your answers as
per 10,000 person-years of follow-up where appropriate.
A. What is the incidence
density of prostate cancer in all participants in the study? (2 points)
B. What is the incidence
density of prostate cancer in daytime workers? (2 points)
C. What is the incidence
density of prostate cancer in fixed night workers? (2 points)
D. What is the incidence
density of prostate cancer in rotating shift workers? (2 points)
E. What is the ratio of the
person-time incidence (or incidence density) of prostate cancer in fixed night
shift to the person-time incidence of prostate cancer in Daytime workers? (2
F. What is the ratio of the
person-time incidence of prostate cancer in rotating shift to the person-time
incidence of prostate cancer in Daytime workers? (2 points)
G. How do rates of prostate
cancer among fixed night and rotating shift compare to the rate of prostate
cancer in daytime shift? What would you conclude about the relationship between
work schedule and risk or rate of prostate cancer? Why? (2 points)
H. Assuming that all participants
were followed up for a total of 8 years with complete follow-up, what would be
the cumulative incidence (risk) of prostate cancer in daytime workers for the
8-year period? (2 points)
(1) Below are excerpts from a study of Urinary Tract
Infections (UTI) in hospitals by Reintjes et al. (Epidemiology 2000;11:81-83).
Background – Severijnenet al. conducted a
prospective multicenter study in eight hospitals to determine the feasibility
of standardized surveillance of nosocomial infections in The Netherlands. After
this study had been completed, the authors used the dataset from gynecologic
patients to measure the influence of possible risk factors on the development
of urinary tract infections (UTI). UTI are known to be associated with a
variety of risk factors (host and intervention related). These factors are not
independent, and confounding is an obvious potential problem. Antibiotic
prophylaxis has been shown to be effective in randomized clinical trials and is
sometimes used to prevent UTI..ut.ovid.com/gw1/ovidweb.cgi#39#39″> In non-experimental studies antibiotic
prophylaxis has been shown to be associated positively with hospital acquired
infections..ut.ovid.com/gw1/ovidweb.cgi#41#41″> The authors studied the association between
UTI and antibiotic prophylaxis using univariate (crude) and stratified
1 shows the results from the univariate (crude) analysis and Table 2 (next
page) shows the results from a stratified analysis.
Week 8: Data Interpretation and Confounding Exercise.0/msohtmlclip1/01/clip_image002.jpg”>
a relative risk of UTI for antibiotic prophylaxis for the univariate (crude)
results presented in Table 1 (2 points).
does the RR suggest about the effect of antibiotic prophylaxis on the risk of
UTI? (2 points)
Week 8: Data Interpretation and Confounding Exercise.0/msohtmlclip1/01/clip_image004.gif”>
relative risks for the 2 strata of hospitals presented in Table 2 (4 points).
do the results from the crude and stratified analysis compare? Is prophylaxis treatment protective? (5 points)
findings are consistent with what you would expect? Which findings do you believe and why? (5 points)
Epidoria Birth Weight Study
Background –In the small island nation of
Epidoria, a team of reproductive epidemiologists has been studying the
relationship between very low birth weight and risk of cognitive, motor, and
behavioral problems. Five years ago these investigators initiated a study.
Using birth certificate files and delivery room entry logs, these investigators
attempted to identify all full-term births in Epidoria over a 6-month period.
The investigators enrolled all low birth weight babies and a representative
sample of normal birth weight babies into their study. The investigators then
examined the children every year until age 3 years. During the last
examination, the investigators administered a standardized developmental
screening test to assess personal-social, language, and motor-adaptive skills.
Based on this test, the investigators classified the children into two groups:
normal development and delayed development.
results from the study were:
a) What kind of a study is this (1 point)? What is the exposure under
study (1 point)? What is the outcome
under study (1 point)?
b) Calculate the crude cumulative
incidence ratio (relative risk) (2
(cont.)– To account for
the possibility that environmental lead exposure might confound the
relationship between birth weight and developmental status, blood lead levels
were determined from blood samples collected at the age 3-year visit. Elevated
lead levels (> 10 micrograms/ dL) were found in 173 of the low birth weight
children (88 of whom had delayed development according to their screening
test). Elevated lead levels were also found in 72 of the normal birth weight
children (24 of whom had delayed development).
out a stratified analysis of birth weight and developmental delay, controlling
for blood lead level. Create 2×2 tables
for each stratum of lead level and estimate the relative risk for each stratum (10 points).
the findings from the crude and stratified analysis, is there a suggestion of
confounding by lead exposure in this example (2 points)?
e) Is there evidence of an association
between blood lead level and the primary exposure variable in this study (hint
create a 2×2 table for blood lead level and exposure) (6 points)?
f) Is there evidence of an association
between blood lead level and the outcome in this study (6 points)?
g) Based on all of this information would
you judge blood lead level to be a confounder of the association between birth
weight and delayed development? Explain your answer. (3 points)
h) What are two changes in the study
design would have avoided the potential confounding effects of blood lead
level? What are the advantages and
disadvantages of these alternatives (5