University of North Carolina at Chapel Hill
                          School of Public Health
                         Department of Epidemiology
                   Fundamentals of Epidemiology (EPID 168)

                       Final Examination, Fall 1996
                               Answer Guide

   1.  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.

   2.  A. Manifestational criteria 

   3.  C. Information bias

   4.  C. Provide an estimate of the dietary exposure in source population from
          which the cases arose.

   5.  A. Controls underreported sucrose intake but cases did not.

   6.  B. This differential selection bias would underestimate the odds ratio.

   7.  Validity refers to accuracy or how well an instrument or method measures
       what it purports to measure.  Reliability refers to repeatability, does
       an instrument or method get the same result or answer consistently,
       regardless of whether the reading is correct.

   8.  A. allows examination of rare diseases.

   9.  D. Ratio (The response scale for each item was ordinal, but in order to
          create the total energy variable the authors had to convert each
          response into calories.)

  10.  B. Matching plus mathematical modeling.

  11.  The odds ratios for 80 to 104 grams per day was 1.4 and for intakes of
       greater than 105 grams per day the odds ratio was 1.3.  This suggests a
       tendency for cases to have a greater proportion of high fat eaters than
       controls.  However, the confidence intervals are broad, extending as low
       as 0.4 and 0.6. Furthermore there is no suggestion of a dose response.
       This is at most weak evidence of a relationship between fat intake and
       ulcerative colitis.

  12.  a. The crude (with respect to smoking) and adjusted odds ratios are the
          same.  If smoking had been a confounder in the relationship between
          sucrose and Crohn's disease or between fiber and Crohn's disease the
          adjusted odds ratio would have been meaningfully different from the
          values in Table 2.

  13.  a. Odds ratios:   Crude = (24 x 285) / (20 x 128) = 6840 / 2560 = 2.7
          among High education = (10 x 150) / (12 x 100) = 1.3
          among  Low education = (14 x 135) / (28 x 8) = 8.4 

       b. The stratum-specific odds ratios are quite different from each other,
          suggesting some degree of effect modification.  The crude odds ratio
          is within the range of the two stratum-specific odds ratio, which
          suggests that education is not so much a confounder as an effect

       c. Three conceptual models of the relationship among fast food, education
          and Crohn's disease could be:

          education-- lower fast food-- lower Crohn's disease (i.e., higher
          educational status could lead to lower fast food consumption which
          could then lead to reduced association with Crohn's disease)

          education-- (lower fast food + education) -- lower Crohn's disease
          (i.e., education also has an interactive effect with fast food
          consumption to lead to an association with Crohn's disease)

          [education -- lower Crohn's disease] AND [lower fast foods-- lower
          Crohn's disease risk] (i.e., lower fast food intake and education act
          as independent main effects to influence Crohn's disease risk).

  14.  A. Nondifferential misclassification bias

  15.  B. A confidence interval provides information on the precision of the
          point estimate.

  16.  There appears to be a strong protective effect of daily consumption of
       Muesli-type breakfast cereals and Crohn's disease (odds ratio = 0.2 [0.1-
       0.7]).  The association is considerably weaker for weekly consumption of
       these cereals (odds ratio = 0.8).  There is evidence of a dose response
       relationship, even though the OR for weekly consumption was not
       statistically significant.  One should also consider that the absolute
       number of cases with daily consumption of Muesli-type cereals is small

  17.  The authors state that sucrose and fiber intake could be associated with
       one another as well as with Crohn's disease and thus each factor might be
       a confounder of the associations between Crohn's disease and the other
       ("mutual confounding").  The odds ratio was 2.6 for a high sucrose intake
       (bottom page 48).  When adjusted for fiber the sucrose odds ratio changed
       only slightly to 2.5.  Therefore, fiber was a only a slight modifier of
       the sucrose and Crohn's disease relationship. 

  18.  a. Under the additive model, we expect the joint excess rate of the two
          factors will be equal to the sum of the excess rate from each factor
          separately.  The additive model can also be written in terms of rates:
          expected rate of ulcerative colitis with both daily soft drink and =2
          fast foods per week = rate (daily soft drinks, without fast food) +
          rate (less freq. soft drink, =2 fast food per week) - rate (neither). 

          If Ri,j is the rate for exposures i and j = 1 (present) or 0 (absent),
          then the additive model is: R1,1 = R1,0 + R0,1 - R0,0.  This equation
          expressed with numbers from the tables is:  Expected joint rate = 9.1
          + 6.8 - 3.7 = 12.2.  The observed rate with both factors was 18.0.
          Therefore, the additive model does not explain the full amount of the
          observed joint risk.

          Under the multiplicative model, we expect the joint rate ratio of the
          two factors to be equal to the product of the rate ratios for each
          factor separately.  In the above notation, the model can be expressed
          as: R1,1 = (R1,0 x R0,1)/R0,0.  This equation expressed with numbers
          from the tables is: (9.1 x 6.8) / 3.7 = 16.7.  The observed rate is
          18.0.  The close agreement for the observed joint rate and that
          expected under the multiplicative model suggests that the relationship
          among daily soft drink consumption, frequent fast food exposure, and
          Crohn's disease is closer to multiplicative than to additive.

       b. Generally, synergism from a public health perspective is equated with
          a joint effect that is greater than expected with an additive model.
          Therefore, the relationship between fast foods and soft drink is
          synergistic, implying that the exposure group to target for maximum
          reduction in ulcerative colitis rates per person year is people who
          consume both fast foods and soft drinks.  One could propose posting
          warnings signs in fast food establishments, soft drink vending
          machines, and beverage containers, etc.

  19.  odds ratio for =3 caffeinated coffee = (56 x 36) / (18 x 36) = 3.1
       Heavy caffeinated coffee drinking now appears to be a risk factor for
       Ulcerative colitis where before coffee drinking appeared to be
       protective.  An alternative approach would be to include the
       decaffeinated coffee drinkers in the "No" (caffeinated) coffee group.
       Under this model the odds ratio for =3 cups caffeinated coffee, relative
       to none or only decaffeinated = (56 x 201) / (50 x 18) = 12.5

  20.  B. Gender -- all subjects in this analysis are women.

  21.  Regression coefficient = log (OR) = log (0.7) = -0.36

  22.  236 cases / 5,000,000 person years = 4.72 cases/100,000 person years.
       Full credit was given for 236 cases / 4,000,000 person years = 5.9 cases
       / 100,000 per year.  Note that the incidence is obtained from all cases
       (or at least all confirmed cases), rather than from only consenting
  23.  There is more confounding for sucrose:
          For sucrose:
          Crude OR = (34 x 67) / (27 x 38) = 2.22, versus adjusted OR of 3.6

          For disaccharides:
          Crude OR = (30 x 66) / (35 x 45) = 1.26, versus adjusted OR of 1.2

  24.  Strengths could include attempts to evaluate dose response, population-
       based case and control selection, validation of case status, large study
       population.  Weaknesses include potential for recall bias, information
       bias in diet assessment.

  25.  Strength of association (This study assessed the strength of association
       by calculating odds ratios.  These measures of strength were also put in
       context by providing confidence intervals.  Some stratum-specific odds
       ratios were strong while others were very weak.), dose response,
       consistency across studies (limited).

  26.  a. F
       b. F
       c. T
       d. F

  27.  Models of joint effects combine effects of "pure" exposures, i.e., in the
       absence of other exposures.  But the excess risk for each food item in
       Table 2 is estimated without controlling for the effects of others.  For
       example, since people who eat fast foods are also likely to take soft
       drinks and not to eat whole grain bread, the relative risk estimates for
       fast food 2+ times/week probably already reflect frequent soft drink
       consumption and low whole grain bread consumption.  In order to add up
       the excess risk for each food item, we need to know the excess risks for
       exposure to that item in the absence of the others.

12/31/96, 1/16/97 \ epid168 \ exams 1996 Final exam - answer guide