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

Midterm Examination, Fall 1999

  1. Which of the following best describes the retrospective design where subjects are sampled by disease status and is often used when the investigator is interested in rare diseases. (4 pts)
    1. intervention trial
    2. case control study
    3. retrospective cohort
    4. ecologic study
    5. none of the above
  1. Which of the following best describes the study design that can be either retrospective or prospective and is often used when the investigators are interested in rare exposures. (4 pts)
    1. intervention trials
    2. cohort studies
    3. prevalence studies
    4. case control study
    5. none of the above
  1. The strength of an association is one of the criteria for evaluating the cause and effect relationship between an exposure and outcome. Which of the following is a measure of the strength of association? (Choose one best answer). (4 pts)
    1. incidence rate among the exposed
    2. cumulative incidence among the exposed
    3. the ratio of odds of exposure among cases to the odds of exposure among the non-cases
    4. odds of disease among exposed relative to the prevalence of exposure in the source population
    5. none of the above
  1. Incidence rates of a disease are often referred to as direct measures of risk. Can incidence rates be calculated from case-control studies? Briefly explain in 1-2 sentences why they can or can not be calculated. (4 pts)
  2.  

  3. For each of the following epidemiological measures, indicate whether it is a rate, a proportion or that it is neither a rate nor a proportion. Circle the best answer. (1 pt each)
a. Population attributable risk

RATE

PROPORTION

NEITHER

b. Incidence density (ID)

RATE

PROPORTION

NEITHER

c. Prevalence

RATE

PROPORTION

NEITHER

d. Relative risk

RATE

PROPORTION

NEITHER



  1. Indicate true or false next to each of the following. (2 pt each)
____ ____ a. A "J" or "U" shaped relationship of a continuous risk factor and continuous measure of disease suggests a Pearson product-moment correlation coefficient of near plus one or minus one.
________b. A risk ratio measure and a correlation coefficient are both measures of association.
________c. A population attributable risk proportion depends on the prevalence of exposure and is not directly related to the strength of an association.
________d. The study base for a case-control study consists of those people who if they developed the disease could have been counted as cases.
________e. The Bradford Hill criterion "coherence" means that the association has been observed repeatedly in different places, by different observers, and at different times.
________f. If an exposure is a cause of a disease, then "temporality" is the Bradford Hill criterion for causal inference that must hold true between exposure and disease.

  1. The death rates from various conditions are often compared across geographic areas. These comparisons are usually based on directly age-standardized mortality rates. Which of the following best describes what is meant by an age-standardized rate created by the direct method? (Choose one best answer). (4 pts)
    1. The number of events in each age stratum of a standard population is used to create a weighted average rate.
    2. The event rates in each age stratum in the standard population are used to create a weighted average rate.
    3. 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.
    4. The event rates in the geographic area of interest are compared to the event rates of a standard population to create a summary rate that is a weighted average.
  1. In order to estimate counts and rates of work-related fatalities, the National Traumatic Occupational Fatality system has introduced a tick-box on the death certificate to indicate "injury at work." Kraus et al. (Am J Epidemiol 1995; 141: 973-9) attempted to validate this "injury at work" classification system against a gold standard [International Classification of Diseases (ICD) death certificate codes designating deaths that occurred during work-related activities]. After reviewing a sample of 100,000 death certificates, the authors reported the following: 1,195 true positives; 788 false positives; 97,672 true negatives; 345 false negatives. ("positive" indicates that the tick-box was checked; "negative" indicates that it was not checked; "true" indicates agreement between the tick-box and the ICD code).
    1. Using the counts provided above, complete the 2x2 table below: (2 pts)

 

ICD Classification

Death Certificate

Work-related

Not work-related

TOTAL

Work-related

 

 

 

Not work-related

 

 

 

TOTAL

 

 

 

    1. What are the sensitivity and specificity of the "injury at work" classification system? (4 pts)


    2. What is the positive predictive value? In your own words, how would you interpret this value? (3 pts)


    3. Based on these data is the death certificate "injury at work" classification system likely to underestimate or overestimate the true number of work-related fatal injuries? (2 pts)


    4. The use of data from the "tick-box" on the death certificates to track work-related mortality trends is an example of which kind of surveillance system? (choose one best answer). (4 pts)
      1. Active surveillance
      2. Passive surveillance
      3. Retrospective cohort surveillance
      4. Cross-sectional survey surveillance

    1. The sensitivity and specificity computed above are quantitative measures of which of the following aspects of death certificate classification of work-related fatalities? (choose one best answer). (4 pts)
      1. Reliability of death certificate classification
      2. Repeatability of death certificate classification
      3. Validity of death certificate classification
      4. Attributable risk of work-related classification on death certificates
      5. None of the above
  1. Age-related maculopathy is a leading cause of blindness among people 65 and older in the United States, and is estimated to affect between 16 and 26% of people in this age group. In a recent study by Klein, residents aged 43 to 86 years in the town of Beaver Dam, Wisconsin were asked to participate in a study to determine whether cigarette smoking was related to age-related maculopathy. At a baseline examination, participants were asked to report their lifetime smoking habits. After 5 years, participants had an examination to determine whether they had developed age-related maculopathy. The following table presents the number of cases of age-related maculopathy measured at the follow-up examination among the 1232 male participants ages 43-86 who did not have age related maculopathy (ARM) at the baseline examination:

Smoking status

N

Cases of ARM

Never smokers

368

26

Ever smokers

864

79

    1. Which of the following best describes the research design used by in this study? (choose one best answer) (3 pts)
      1. Population based cross-sectional study
      2. Case cohort study
      3. Nested case control study
      4. Prospective cohort study
      5. None of the above
    1. Create a 2 x 2 table where one axis is smoking status and the other is age-related maculopathy status. (4 pts)



    2. Calculate the 5-year cumulative incidence of age-related maculopathy in ever smokers, and in never smokers. Show your work. (4 pts)



    3. Calculate the cumulative incidence ratio comparing the incidence of age-related maculopathy in ever smokers with that in never smokers. Show your work. (4 pts)



    4. Assuming causality, what is the proportion of cases of age-related maculopathy that could have been prevented in the population of males ages 43-86 in Beaver Dam if the smokers had never smoked? Show your work. (4 pts)

  1. The following data come from a national survey of the occurrence of back pain. A case of low back pain was defined as having at least one episode of severe back pain occurring over a period of 6 months. The number of cases was obtained from surveys of different occupation groups as well as a national random sample.

 

Cell phone manufacturing

Textile manufacturing

National random sample

Age

Persons

cases

Rate

Persons

Cases

Rate

Persons

Cases

rate

25-39

1000

2

.002

100

2

.02

10,000

30

.003

40-55

700

25

.037

500

30

.06

15,000

900

.06

55+

50

15

.300

1500

150

.100

15,000

1200

.08

Total

1750

42

.024

2100

182

.087

40,000

2130

.053

    1. Compute a standardized event ratio (similar to a standardized mortality ratio (SMR) except the episodes of back pain arenít mortal events) of back pain for the cell phone-manufacturing employees. Briefly state in one sentence the interpretation of this measure in this case. (3 pts)


    2. Compute a standardized event ratio (similar to a standardized mortality ratio (SMR) except the episodes of back pain arenít mortal events) of back pain for the textile-manufacturing employees. Briefly state in one sentence the interpretation of this measure in this case. (3 pts)


    3. Can these two ratios in part (a) and (b) be compared? Briefly explain why or why not. (3 pts)
  1. The evidence supporting obesity as a risk factor for colon cancer remains inconclusive, especially among women. A recent study (Am J Epidemiol 1999;150:390-398) reported the association between obesity (measured at baseline) and colon cancer morbidity as determined from review of medical records and death certificates in a nationally representative cohort of men and women age 25-74 years who participated in the First National Health and Nutrition Examination Survey from 1971 to 1975 and were subsequently followed up through 1992. The following table is from this study for men and women combined.

Baseline body
mass index*

Number of incident cases of colon cancer

Person-years
of follow up

Crude incidence rate/100,000 PY

<22

28

53,475

 

22 - <24

41

38,919

 

24 - <26

36

36,610

 

26 - <28

40

32,635

 

28 - <30

35

21,122

 

30+

42

34,904

 

* kg body weight per height in meters squared

    1. Which of the following best describes the research design used in this study? (choose one best answer). (2 pts)
      1. Cross-sectional survey
      2. Ecological study
      3. Population based case control study
      4. Cohort study
      5. None of the above
    1. Complete the table by calculating the crude body mass index-specific incidence rates. (3 pts)



    2. Calculate the relative risk (RR) of colon cancer associated with a BMI of 28-<30. Use the lowest BMI category as referent. In one sentence interpret your answer. (2 pts)



    3. Calculate the attributable risk proportion of those in the 28-<30 BMI category. In one sentence interpret your answer. (the attributable risk formulas provided in class can be used even though the data provide is for rates) (2 pts)


  1. Analyses of data from cohort studies often have to deal with the reality that participants have unequal lengths of follow up. Given the data below, calculate the (a) total person time (month) of follow up, (b) the overall incidence density rate, (c) 13 month cumulative incidence, and (d) the product limit estimate of failure. Each horizontal line represents a cohort participant. Each vertical line represents one month. Arrows indicate time of loss to follow up. Black boxes indicate onset of disease (failure). (2 pts each)
    1. ______________
    2. ______________
    3. ______________
    4. ______________

(Unfortunately the diagram is not yet available)

 

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