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

  ____  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

    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.


               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. ___________________________________________________________






10/12/97, 10/13/97   EPID 168 Midterm, Fall 1997
Last changed 4/10/1999 by