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

Midterm Examination, Fall 1999

Answer Guide

  1. B. Case-control studies are said to use sampling by disease and are suited for studying rare diseases.
  2. B. Cohort studies can be either retrospective or prospective and are often used to study rare exposures.
  3. The ratio of odds of exposure among cases to odds of exposure among noncases is the odds ratio, which is a measure of association.
  4. Incidence rates cannot be estimated from case-control studies without additional information. In the case-control design selection of subjects is based on disease status, so the number of cases is under the control of the investigator. If the investigator has access to all cases and knows the size of the population from which they arise s/he can estimate incidence, but knowledge of the population size is not available from the case-control design.

    1. Population attributable risk (PARP)
      Both "proportion" and "neither" received credit, since this is a subtle distinction. According to Regina Elandt-Johnson (Am J Epidemiol 1975;102:267-271), a proportion is a type of ratio in which the numerator is included in the denominator [p=a/(a+b)]. Since PARP can be expressed as ("attributable" cases / all cases), it is indeed a proportion. However, it can also be expressed as a difference of two proportions (I-I0) or the product of a proportion (prevalence) and the difference of two proportions [p(I1-I0)], so it is easy to be misled about its mathematical form (indeed, the "official" answer to this question could not explain why it is a proportion!).

    2. Incidence density (ID) is a RATE.
    3. Prevalence is a PROPORTION.
    4. Relative risk is NEITHER a rate nor a proportion.

  1. Indicate true or false next to each of the following. (2 pt each)
    1. FALSE A Pearson product-moment correlation coefficient measures the extent to which a relationship is linear, so a value of plus one or minus one corresponds to a straight line.

    2. TRUE A risk ratio measure and a correlation coefficient are both measures of association.

    3. FALSE A population attributable risk proportion depends on the prevalence of exposure and is ALSO directly related to the strength of an association.

    4. TRUE The study base for a case-control study consists of those people who if they developed the disease could have been counted as cases.

    5. FALSE The Bradford Hill criterion "coherence" means that all of the known facts about the relationship fit into place; the criterion of "consistency" means that the association has been observed repeatedly in different places, by different observers, and at different times.

    6. TRUE "Temporality" is the one Bradford Hill criterion for causal inference that must hold true between exposure and disease.

  1. C. "The event rates in the geographic area of interest are applied to the age-stratum sizes of a standard population to create a rate that is a weighted average" describes a directly-standardized rate.
  2.     a.


ICD Classification


Death Certificate


Not work-related






Not work-related








    1. Sensitivity = 1,195/1,540 = 78% Specificity = 97,672/98,460 = 99%

    2. Positive predictive value = 1,195/1,983 = 60%

    3. Based on these data the death certificate "injury at work" classification system will overestimate the true number of work-related fatal injuries, since more non-work-related injuries will be classified as work-related than vice-versa.

    4. B. Passive surveillance the reports are submitted by health care workers in conformance with a general obligation rather than in response to a specific request from the surveillance organization.

    5. C. Sensitivity and specificity are measures of validity, since there is a standard for "truth".
  1. D. Prospective cohort, since the investigators monitored people without the condition over time to detect its development.



Cigarette smoking status



Ever smokers

Never smokers



ARM cases














    1. CI in ever smokers = # new cases / population at risk = 79/864 = 0.091 in 5 years
      CI in never smokers = # new cases / population at risk = 26/368 = 0.071 in 5 years
    2. (was labeled "e") Cumulative incidence ratio (CIR) = CI in ever smokers / CI in never smokers
      = (79/864) / (26/368) = 1.29

    3. (was labeled "f") PARP = (overall incidence incidence in never smokers) / overall incidence of ARM
      = (0.0852 0.0707) / 0.0852 = 17%
    1. Standardized event ratio (for cell phones) = SMR (cell phone) = observed/expected
      = 42/{(.003)(1000) + (.06)(700) + (.08)(50)} = 42/49 = 0.86

    2. Standardized event ratio (textiles) = SMR (textile) = observed/expected
      = 182/{(.003)(100) + (.06)(500) + (.08)(1500)} = 182/150 = 1.2

    3. These two ratios cannot be compared directly. An SMR is a weighted average where the weights (e.g., age structure) come from the population for which indirect standardization is being carried out. So SMRs for two populations use different weights. Unless the populations have identical age structures, the stratum-specific rates are the same for all strata, or the stratum-specific rates for one population are a constant multiple of those for the second population, the comparison is invalid. With indirect standardization, it is actually the "standard population" rates that are being "standardized" to the age distribution of the study population.
Baseline body mass index*

Number of incident cases of colon cancer

Person-years of follow up

Incidence rate/100,000 PY





22 - <24




24 - <26




26 - <28




28 - <30








* kg body weight per height in meters squared

    1. D. Cohort study

    2. RR of colon cancer for BMI 28-<30 kg/m2 vs. lowest = 165.7/52.4 = 3.16

    3. ARP for BMI 28-<30 kg/m2 vs. lowest = (3.16 1) / 3.16 = 68%

      The ARP of 68% means that 68% of the incidence in the 28-<30 kg/m2 group is attributable to elevated BMI.
    1. 43 person-months

    2. 3 cases/43 person-months = 7.0 cases per 100 person-months

    3. 13-month CI = 3/7 = 0.43

    4. Product-limit estimate of survival = 1-[(6-1)/6 x (5-1)/5 x (3-1)/3)] = 1-0.444 = 0.555


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