University of North Carolina at Chapel Hill
School of Public Health
Department of Epidemiology
Fundamentals of Epidemiology (EPID 168)
Midterm Examination, Fall 1997
NOTE: Adjust margins and/or pagination before printing.
NOTE: This exam is illustrative only. It proved somewhat on the easy side, and a number of the questions were problematic.
1. Match the term from column A with the most appropriate topic or
concept from column B (use each term only once and each topic only
once). (1 pt each = 12 pts)
Column A - Terms Column B - Topics
____ cumulative incidence 1. Case-control studies
____ incidence density 2. Causal inference
____ prevalence 3. Confounds cross-sectional data
____ dose response 4. Death certificate
____ induction period 5. Descriptive epidemiology
____ odds ratio 6. Diagnostic tests
____ preventive fraction in the exposed 7. Estimates risk
____ underlying cause of death 8. Measures impact
____ positive predictive value 9. Natural history of disease
____ detectable, pre-clinical phase 10. Population screening
____ migrant studies 11. Proportion
____ cohort effect 12. Relative rate
2. Which of the following best describes the basis of the diagnosis of
myocardial infarction? (Choose one best answer) (4 pts)
____ a. manifestational criteria
____ b. Bradford criteria
____ c. causal criteria
____ d. etiologic criteria
3. In the Minnesota Heart Health Program (as described in class) and many
other community intervention studies, the effectiveness of an
educational intervention program is evaluated. Which of the following
selections best describes the unit of assignment, the unit of
observation, and the unit of analysis (in this order) in studies of
these types? (Choose one best answer) (4 pts)
____ a. community, person, community
____ b. person, community, community
____ c. community, community, community
____ d. none of the above
-2- ID Number __-__ __ __ __
4. In a hypothetical clinical trial, a new drug was compared with
"standard therapy" treatment. The endpoint was myocardial infarction.
Which of the following best describes the primary reason to randomize
patients to treatments? (Choose one best answer) (4 pts)
____ a. to create two treatment groups that are similar at baseline on
both known and unknown factors associated with myocardial
infarction.
____ b. prevent bias introduced when the patients know what type of
treatment they are receiving
____ c. prevent bias introduced when the investigators know what type of
treatment the patients are receiving
____ d. b and c
5. Indicate TRUE or FALSE next to each of the following statements.
(2 pts each)
____ a. The indirect method of age standardization applies stratum-
specific rates from an external population to the age distribution
of the study population.
____ b. A standardized mortality ratio is an example of a stratum-specific
crude rate.
____ c. Standardized mortality ratios are perferred for making comparisons
among multiple populations.
____ d. Direct age standardization can be characterized as applying the
same set of weights to the age-specific rates of populations to be
compared.
6. 200 women with a history of chest pain were assessed by an exercise
tolerance test (ETT). Compared with coronary angiography (the "gold
standard"), ETT had a sensitivity of 68% for detecting coronary artery
disease, with specificity 61%. The predictive value of a negative ETT
was higher in younger women (less than 52 years old) and in women with
no more than one risk factor (i.e., family history, hypertension, high
cholesterol, smoking, or diabetes). If sensitivity and specificity do
not vary by age or risk factor status, why is the higher negative
predictive expected? (3 pts)
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
-3- ID Number __-__ __ __ __
7. A randomized trial studied 242 HIV-seropositive, 2nd-trimester
pregnant women to assess the efficacy of zidovudine (AZT) in
preventing perinatal HIV transmission. Results were:
Results from a randomized trial of the efficacy of
zidovudine in preventing perinatal HIV transmission
___________________________________________________________________
Zidovudine Placebo All
Births (no.) 121 121 242
Infection status of infant
Non-infected 112 90 202
HIV-infected 9 31 40
Transmission rate (%) 7.4 25.6 16.5
___________________________________________________________________
7A. Which one answer best describes the transmission rate in the table?
(4 pts)
____ a. proportion
____ b. relative rate
____ c. absolute rate
____ d. odds
7B. Using the data in the table, estimate the relative risk of HIV
infection for infants whose mothers took zidovudine relative to
infants of mothers who took placebo. Show formula and calculations.
(4 pts)
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
-4- ID Number __-__ __ __ __
7C. Based on the data in the above table, estimate the proportion of
potential cases of perinatal HIV transmission that could be prevented
by providing zidovudine to HIV-positive, 2nd trimester pregnant women
who would otherwise not receive the drug. (Assume all women take the
medication and consider only singleton births.) Show formula or
diagram and calculations. (4 pts)
7D. Zidovudine is now routinely offered in association with all
pregnancies to known HIV-seropositive mothers in the United States.
However, growth of resistant strains will reduce the drug's
effectiveness in preventing perinatal HIV transmission. Observational
studies for assessing zidovudine's effectiveness have serious
methodologic problems, but which of the following case-control designs
would be the most nearly valid? (Choose one best answer.) (4 pts)
____ a. Cases are HIV-infected infants; controls are uninfected infants.
____ b. Cases are HIV-infected infants; controls are uninfected infants of
HIV-seropositive mothers.
____ c. Cases are HIV-infected infants; controls are infants whose mothers
should have received zidovudine but did not.
____ d. Cases are HIV-infected infants whose mothers received zidovudine;
controls are uninfected infants whose mothers received zidovudine.
8. The following is background information for questions 8A-8E.
Objective: To determine the prevalence of sexually transmitted
diseases (STD) and high risk sexual behavior for STD among adolescent
males admitted to a juvenile detention facility.
Methods: Data were obtained from interview, exam, and lab tests.
Results:
Table 1. Behavioral variables in 966 subjects
___________________________________________________________________
Variable Mean (SD) Range Median
Age at first coitus 12.3 (2.0) 5-17 13
No. lifetime partners 13.7 (16.8) 1-100 8
No. partners past 4 months 2.9 (3.4) 0-30 2
No. weeks since last sex 5.8 (15.1) 1-260 2
___________________________________________________________________
SD = standard deviation
-5- ID Number __-__ __ __ __
8A. Which of the descriptive statistics in Table 1 (mean, SD, range,
median) is most susceptible to being influenced by a single extreme
value? (Choose one_best answer.) (4 pts)
a. mean
b. SD
c. range
d. median
8B. Of the four variables in Table 1, which has the most symmetrical
(normal-like) distribution? (Choose one best answer.) (4 pts)
a. age at first coitus
b. number of lifetime partners
c. number of partners in the past 4 months
d. number of weeks since last sex
Table 2. Sexually transmitted diseases in adolescent males
admitted to a juvenile detention facility.
______________________________________________________
No. positive
Disease /tested
Syphilis 7/930
Gonorrhea 42/940
Chlamydia 66/957
Any of the above 109/908
_______________________________________________________
8C. Based on the above data and assuming that the the two diseases have
the same average duration, how do their incidence rates compare in
this population? (Choose the one correct answer.) (3 pts)
a. Incidence of gonorrhea is lower than that of chlamydia.
b. Incidence of gonorrhea is the same as that of chlamydia.
c. Incidence of gonnorhea is higher than that of chlamydia.
-6- ID Number __-__ __ __ __
8D. Based on the above data but this time assuming that the two diseases
have the same incidence, how do their average durations compare in
this population? (Choose the one correct answer.) (2 pts)
a. Duration of gonorrhea is shorter than that of chlamydia.
b. Duration of gonorrhea is longer than that of chlamydia.
8E. Elaborate on your answer to the preceding question by deriving an
estimate of the relative duration of gonorrhea relative to chlamydia.
Show the basis for your answer. (3 pts)
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
9. The following is background information for questions 9A-9D.
In a large urban school district, among 8,000 middle-school school
youth who were well at the beginning of the school year, 400 were
absent for 10 days or longer due to acute asthma ("AA-10") during the
first nine-week quarter. Based on a survey believed accurate for the
period, 15% of middle-school youth in the county middle schools smoke
cigarettes. Interviews with the youth who were absent for 10 days or
longer revealed that 100 of them were cigarette smokers. Assume that
the school enrollment does not change during the quarter.
9A. Show these data in the form of a 2 x 2 table. Include an appropriate
title, labels that identify each row and column, and row and column
totals. (4 pts)
9B. What is the cumulative incidence (CI) of AA-10 (10+ absent days due to
acute asthma), in:
a. the cohort of 8,000 youth? (1 pt)
b. youth who smoke cigarettes? (1 pt)
c. youth who do NOT smoke cigarettes? (1 pt)
-7- ID Number __-__ __ __ __
9C. What measure would you use to quantify the strength of association
between cigarette smoking and AA-10? Show the formula for this
measure, substitute the appropriate numbers for that formula, compute
the result, and state its meaning in one sentence. (4 pts)
a. Formula
b. Substitution
c. Result
d. Meaning ____________________________________________________
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
9D. Assuming that cigarette smoking is responsible for the observed excess
in AA-10, how many cases of AA-10 during the quarter are attributable
to cigarette smoking? Show a relevant formula or diagram,
intermediate computation, and result, and give a sentence stating the
meaning of the result. (4 pts)
a. Formula or diagram
b. Substitution
c. Result
d. Meaning ____________________________________________________
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
-8- ID Number __-__ __ __ __
10. Suppose that 900 of the subjects in question #8 consent to regular STD
screening following release from detention. Subjects are counseled
about preventive measures and screened every three months for two
years. All cases are treated and cured.
Table 3. Numbers of cases of three sexually transmitted diseases
in adolescent males discharged from a juvenile detention facility
____________________________________________________________________
Follow up Time (Months)
3 6 9 12 15 18 21 24
Syphilis 0 1 0 3 1 2 3 4
Gonorrhea 10 8 15 21 11 12 19 24
Chlamydia 15 23 8 18 17 17 14 11
Dropouts (cumulative) 10 30 50 90 120 140 190 270
Number tested 890 870 850 810 780 760 710 630
____________________________________________________________________
(Subjects can become infected with the same organism more than once
and/or become co-infected with more than one organism.)
10A. What is the prevalence of chlamydia at the 12 month follow-up? (3 pts)
10B. What is the average incidence density (per 100 person months or per
100 person years) of chlamydia for the two years of follow up? Assume
that: dropouts contribute no time to follow up after the last time
they are tested; subjects remain at risk even while infected. (3 pts)
10C. Give two reasons for preferring incidence density over cumulative
incidence for assessing frequency of infection in this cohort. (6 pts)
i. ___________________________________________________________
_______________________________________________________________
ii. ___________________________________________________________
_______________________________________________________________
-9- ID Number __-__ __ __ __
11. A study of alcoholism and major depressive disorder recruited 100
consecutive patients in a Veterans Administration hospital in Urbana,
Illinois. All patients had been diagnosed as being alcohol abusers.
An equal number of non-abusers were selected randomly from the same VA
hospital. 76 of the participants identified as being abusers
fulfilled criteria for major depression, as did 20 of the non-abusers.
Evaluate the evidence provided by this study for the inference that
alcohol abuse causes depression in relation to the following aspects:
11A. What is an inherent weakness in this design that makes it susceptible
to obtaining inaccurate data? (3 pts)
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
11B. Many of the criteria for causal inference pertain to the evaluation of
evidence from multiple studies, but several can also apply to a single
study. Name two (2) such criteria and use them to evaluate
(quantitatively where possible) the evidence from the above study.
(6 pts)
i. ___________________________________________________________
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
ii. ___________________________________________________________
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
Congratulations!
10/12/97, 10/13/97 EPID 168 Midterm, Fall 1997
Last changed 4/10/1999 by Victor_Schoenbach@unc.edu