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Notes workgroups Epidemiology (AM_1179)
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These are notes from all the workgroups given in Epidemiology, including all questions and answers that we discussed.
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Workgroup 1 - 09/01/2023l.m.molenaar@vu.nl
Exercise 1
Read the following passages. Identify the study designs and determine what the outcome and
exposure of interest are. Please justify your choices.
a) Questionnaires were mailed to every 10th person listed in the city telephone directory.
Each person was asked to list age, sex, smoking habits, and respiratory symptoms during
the preceding seven days. About 20% of the questionnaires were completed and
returned. About 10% of respondents reported having upper respiratory symptoms.
Answer: observational -> analytical -> cross-sectional - exposure = frequencies of age, sex,
smoking habits → outcome: respiratory symptoms → exposure and outcome measured at the
same time.
b) A sample of 1,500 employees from an airline company were initially examined in 1951.
They were classified by diagnostic criteria for coronary artery stenosis (CAS). New cases
of CAS have been identified by radiological examination every three years,
complemented by ‘cause-of-death’ certificates. Heart attack rates in different subgroups
have been computed.
Answer: observational -> analytical -> prospective cohort study → outcome: death by heart
attack → exposure: CAS → outcome comes after exposure (outcome is assessed in the future) →
following a group from the present onwards, so assessing the exposure now and continually.
c) A random sample of rural community members was selected from four census tracts, and
each person was examined for depression. All persons without disease were randomly
assigned to either a two-years program of physical activity encouragement or a
two-years program of lifestyle advice.
Answer: experimental -> RCT (because involves manipulation) → exposure: intervention (physical
activity encouragement/lifestyle advice) → outcome: depression.
d) Patients admitted for carcinoma of the stomach and patients without a diagnosis of
cancer are interviewed about their tobacco chewing history. The question is whether
there is an association between tobacco chewing and gastric cancer.
Answer: observational -> analytical -> case-control → exposure: tobacco chewing → outcome:
gastric cancer → starts with the outcome and then looks to determine the exposure → cases are
matched with controls (always retrospective) → also used for rare outcomes.
e) Researchers retrieved census data at regional level, including median wealth index for
the respective regions, from the Ugandan Bureau of Statistics. These data were matched
with air pollution levels per region which they measured over a one-month period. Data
were analysed using geographic information systems (GIS), producing maps showing air
pollution levels per region.
Answer: observational -> descriptive -> ecoligcal → exposure: wealth level by region → outcome:
air pollution → the unit of inference is a region, using ecological variables (air pollution) → nearly
always compare groups at aggregate levels (countries/regions) → usually ecological studies do
not involve primary data collection.
Exercise 2
, You have developed the hypothesis that automobile drivers who regularly sleep less than six
hours a night have a higher incidence of fatal accidents compared to those that sleep longer than
six hours. Think about the design of a case-control study to test this hypothesis. Consider:
a) How will you define a ‘case’? Are there any special considerations?
Answer: outcome = fatal accidents → case = deceased individual with fatal accident as cause of
death → considerations: primary cause of death, driver vs. passenger.
b) Where would you find the cases?
Answer: death certificates (hospital records), medical records, police records (reporting systems
for road accidents).
c) How would you define a ‘control’?
Answer: similar individuals to the case, but without the outcome of interest → automobile
drivers not in an accident/that did not die.
d) Name a suitable population from which to choose the controls for your study.
Answer: automobile drivers not in an accident → drivers from similar areas with comparable
characteristics (age, sex, socioeconomic status, hours of work per week) → random sample from
motor vehicle license records (RDW).
e) What major characteristic must you strive to measure similarly in each study participant
(both the cases and controls)?
Answer: amount of sleep → other characteristics: age, road conditions, weather, sex, medical
history, passenger restraint systems in use, safety features of the automobile.
f) What difficulties could be encountered in measuring these characteristics?
Answer: difficult to measure the amount of sleep (both for cases and controls) → if you ask
relatives with cases, you also have to do this with controls.
Exercise 3
What is the importance of randomisation in an intervention trial, and what does it accomplish?
Answer: randomisation prevents/reduces bias and you accomplish a representative picture of
occurrence of an event in the population → leads to a higher generalisability.
Exercise 4
What is meant by the phrase ‘ecological fallacy’?
Answer: outcomes are not applicable to individual levels → groups of individuals may differ
greatly from the larger group.
Exercise 5
Different study designs have particular advantages and disadvantages. Contrast the case-control
and prospective cohort design with respect to the following factors. a) Costs; b) Time required
for completion of the study; c) Bias; d) What can be estimated (with regards to effect/association
measures);
Answer: (a) cohort is more expensive (because following a group for a long time), (b) cohort is
time-consuming, (c) cohort inferes less bias (case-control has many difficulties and thus chances
for bias), (d) cohort studies allow for exploration of natural developments between exposure and
outcome, whereas case-control studies you retrospectively assume 2 things to be associated.
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