University of North Carolina
School of Public Health
Department of Epidemiology
Fundamentals of Epidemiology (EPID 168)
Victor J. Schoenbach and Wayne D. Rosamond
Fall 1996 Final Exam (Tuesday 10 Dec 1996)
This examination is based on Per-Gunnar Persson, Anders Ahlbom, Goran
Hellers. Diet and inflamatory bowel disease: a case-control study.
Epidemiology 1992;3:47-52.
NOTE: For simplicity, ignore the requirement that this study was
restricted to those persons with a telephone number.
1. Which of the following best describes the primary objective of this
study? (Choose one best answer) (3 pts)
A. To test the hypothesis that persons with inflammatory bowel disease
are more likely to have been exposed to certain dietary factors than
those without inflammatory bowel disease.
B. To test the hypothesis that the risk of having inflammatory bowel
disease given that you have certain dietary exposures is greater than
the risk of not having inflammatory bowel disease.
C. To test the hypothesis that the increase in inflammatory bowel
disease in the population is attributed to certain dietary exposures.
D. To test the hypothesis that the average consumption of certain
dietary factors increases as the proportion of a group of people with
inflammatory bowel disease increases.
2. Designation as a case of ulcerative colitis was based on which of the
following classification models. (Choose one best answer) (3 pts)
A. Manifestational criteria
B. Causal criteria
C. Both manifestational and causal criteria
D. Neither
3. Medical records were used to validate the hospital diagnoses of Crohn's
disease and ulcerative colitis. By using this validation process
instead of relying on hospital discharges coding alone, the authors are
reducing which of the following sources of error? (Choose one best
answer) (3 pts)
A. Selection bias
B. Prevalence-incidence bias
C. Information bias
D. Surveillance bias
_ -2-
4. Controls were selected as a random sample using the population register
of Stockholm County Council. Which of the following best describes the
primary purpose of using a random sample in this study? (Choose one
best answer) (3 pts)
A. Maximize generalizability by obtaining a statistically representative
sample.
B. Select a control group that was as similar as possible to the case
group except for dietary exposures.
C. Provide an estimate of the dietary exposure in the source population
from which the cases arose.
D. Select a control group with dietary habits similar to those in the
population of cases.
5. Dietary exposures were assessed using a questionnaire with
Òretrospective questions aimed at a period of time 5 years in the pastÓ
(page 48). Which of the following situations of misclassification would
make sucrose appear more harmful than it really was? (Choose one best
answer) (3 pts)
A. Controls underreported sucrose intake but cases did not.
B. Cases underreported sucrose intake but controls did not.
C. Both cases and controls underreported sucrose intake.
D. Both cases and controls overreport sucrose intake.
6. Suppose that cases excluded due to administrative delay problems were
more likely to have daily soft drink exposure than less than daily.
Which of the following best describes the impact this would have on the
odds ratio presented in Table 3? (Choose one best answer) (3 pts)
A. Without the exclusion the odds ratio would be closer to the null.
B. Without the exclusion the odds ratio would be larger.
C. The exclusion did not affect the odds ratio.
D. Cannot determine on the basis of this information.
7. Diagnoses of disease were verified in this study. Define validity and
compare and contrast this concept with reliability. (4 pts)
8. This study uses a case control design with a population based control
group. Which of the following, in general, is a strength of this
design. (Choose one best answer) (3 pts)
A. Allows examination of rare diseases.
B. Allows examination of rare exposures.
C. Good for establishing temporality.
D. Good for equalizing on known and unknown confounders.
_ -3-
9. Items on the food frequency questionnaire were mostly in a format with
six response options that ranged from twice per day or more often to
less frequently than once every 2 weeks (pg 48). In deriving values for
daily energy intake, the authors treated the food frequency responses as
which level of measurement? (Choose one best answer) (3 pts)
A. Nominal
B. Ordinal
C. Interval
D. Ratio
10. Control for age in the analyses presented in Table 2 was accomplished
through which of the following methods? (Choose one best answer)
(3 pts)
A. Stratified analysis plus matching.
B. Matching plus mathematical modeling.
C. Restriction without stratification
D. Mathematical modeling and stratification.
11. Based on the data presented in Table 2, is ulcerative colitis associated
with fat intake among men? Give a brief statement to support your
answer. (4 pts)
12. The authors state on page 49 that after controlling for smoking, the
relative risk for CrohnÕs disease among men was 1.9 for a high
consumption of sucrose and 0.7 for a high consumption of fiber. Briefly
explain why based on these data the authors state that smoking did not
confound these associations. (3 pts)
13. The data presented in Table 3 indicate that Crohn's disease is
associated with the consumption of fast foods. Suppose that when
stratified by educational attainment, the resulting data were as
follows:
Educational attainment
High Low
Controls Cases Controls Cases
Fast foods
1+ times/wk 12 10 8 14
None 150 100 135 28
a. Calculate the crude and stratum-specific odds ratios. (3 pts)
b. Is this association between fast food and CrohnÕs disease confounded
by education level? Quantify and briefly explain your answer. (3
pts)
c. Briefly explain in 2 sentences or a diagram how education might fit
into a conceptual model consisting of fast food, education, and risk
of Crohn's disease. (3 pts)
_ -4-
14. In the discussion (page 50), the authors state that Òif the change in
diet is the same in cases as in controls, then the relative risk
estimates would be biased toward unityÓ. This is an example of which of
the following? (Choose one best answer) (3 pts)
A. Non differential misclassification bias
B. Non differential selection bias
C. Differential information bias
D. Differential misclassification bias
15. This articles does not present p-values yet reports 95% confidence
intervals for all odds ratios. Which of the following best describes
what information a confidence interval conveys that a p-value does not.
(Choose one best answer) (3 pts)
A. A confidence interval puts the observed point estimate in the context
of randomness.
B. A confidence interval provides information on the precision of the
point estimate.
C. A confidence interval includes an estimate of the statistical power
of the study.
D. A confidence interval reflects the clinical significance of the point
estimate.
16. The study describes the association of consumption of Muesli-type
breakfast cereal and Crohn's disease (Table 3). Briefly state and
evaluate the strength of the numerical evidence for the association
between Muesli-type breakfast cereals and Crohn's disease. (3 pts)
17. Briefly present the evidence for or against the role of fiber as a
confounder of the association of sucrose intake and CrohnÕs disease. (3
pts)
18. Suppose a follow-up to this study was done to estimate the rate (per
10,000 person years) of ulcerative colitis among a large sample in the
Swedish population. The table below summarizes the results.
Fast food intake
Soft drink intake 2/week None
Daily 18.0 9.1
Less frequently 6.8 3.7
a. Which model for the joint effect of these two food items, the
additive model or the multiplicative model, better fits the data?
Your answer should give the formula for each model and show how to
evaluate it with the above data. (5 pts)
b. Do these data, assuming that they accurately reflect causal effects,
indicate a synergistic effect from a public health perspective?
Justify your answer and state an appropriate public health
implication if any. (2 pts)
_ -5-
19. This study did not differentiate between caffeinated and decaffeinated
coffee. Using the data presented in Table 4 and applying the
assumptions below, calculate the odds ratio (heavy versus no use)
associated with caffeinated coffee consumption and determine if it is
protective against ulcerative colitis. Describe in 2 sentences or less
the interpretation of this new odds ratio, ignoring issues of random
error. (4 pts)
Assumptions:
1. 20% of the heavy coffee drinkers ( 3 cups per day) among cases drink
only decaffeinated coffee.
2. 90% of heavy coffee drinkers among controls drink only decaffeinated
coffee.
20. Which of the following variables was NOT in the multiple logistic model
that was used to estimate the relative risk for sucrose intake in
relation to ulcerative colitis in women? (Choose best answer) (3 pts)
A. Age
B. Gender
C. Total energy intake
D. Ulcerative colitis
21. In the multiple logistic model that yielded the relative risk estimate
of 0.7 for Ulcerative colitis in relation to daily vegetable consumption
(Table 4), what was the value of the coefficient for the vegetable
consumption variable assuming that it was coded as 1=daily, 0=less
frequently? Write the conversion equation of coefficient to relative
risk estimate. (3 pts)
22. Assume that the population of Stockholm County in the age range covered
by this study was 1,000,000 in 1980 and remained constant throughout the
decade. What was the average annual incidence of hospital-diagnosed
Crohn's disease during that period regardless of when their medical
record became available? (3 pts)
23. Using the data in Table 2, for which of the following two associations
is there more of an indication of confounding by age and total energy
intake in WOMEN? Support your answer with relevant data and/or
computations. (3 pts)
a. Crohn's disease and sucrose intake (highest versus lowest level)
b. Crohn's disease and disaccharide intake (highest versus lowest level)
24. Briefly state one major strength and one major limitation of this study
(2 pts)
_ -6-
25. List two Bradford Hill criteria for evaluating whether dietary sucrose
intake is causally related to inflammatory bowel disease. Evaluate each
using specific facts from the article. (4 pts)
26. Which of the following statements about the data in Tables 1 and 2 are
TRUE and which are FALSE (answer TRUE or FALSE for each statement). (2
pts each)
a. In women, the rate of (hospitalized) ulcerative colitis was higher
than that of (hospitalized) Crohn's disease.
b. The similarity in age distribution between the case groups and
controls indicates that the rates of these disease are fairly uniform
between the ages of 15 and 79 years.
c. Reporting of dietary intake by the Crohn's disease cases involved
recall over longer periods of time, on the average, than was the case
for the ulcerative colitis cases.
d. The proportion of controls with high dietary fat intake was higher
for men than for women.
27. A Swedish friend of yours who lives in Stockhom has an indentical twin
sister who is anything but identical in terms of her diet. Your friend,
as other health conscious Swedes, avoids fast foods and soft drinks, and
eats whole grain bread and muesli-type cereals daily. Her twin sister,
and many Swedes, often consumes fast foods and soft drinks, but never
touches whole grain bread or muesli.
Your friend comes to visit with you over the holidays, and while you are
sleeping late one morning she comes across your class notes from EPID
168. At breakfast, where she has been busily scribbling on her napkin,
she asks you this question.
"Suppose that fast foods, soft drinks, whole grain bread, and muesli-
type cereal affect Crohn's disease risk independently, and that I can
ignore other risk factors. Suppose also that the excess risks are
additive. Is my twin sister's risk of Crohn's disease 10 times my own?"
She shows you how she used the information in Table 3 to obtain that
estimate:
(3.4 - 1) + (2.8 - 1) + ((1/0.4) - 1) + ((1/0.2) - 1) + 1 = 10.7
She goes on to explain "(3.4 -1) is the excess risk from fast foods, and
((1/0.4) - 1) is the excess risk from eating bread that is not whole
grain."
Even though you're not quite fully awake, you feel justifiable pride in
your command of epidemiologic concepts and explain to her the one big
mistake she has made. You say, " . . . ". Write a brief statement of
what you would say. (4 pts)
Rosemond/Schoenbach, 12/96, edited 11/11/97 epid168 \ exams 1996 Final exam