CDC Excercise
Question 1: What makes the first study a case-control study?
Case-control studies have two groups. Cases are groups of individuals with the disease.
Controls are a group of individuals without the disease. The first study is a case-control because
patients with lung cancer (cases) are compared to patients with other disorders (controls).
Compared to a cohort study, case-control studies compare groups of people with the disease to
people without the disease.
Question 2: What makes the second study a cohort study?
In a cohort study there are two groups. The exposed group has been exposed to the factor that
could potentially cause the disease. The unexposed group has not bee exposed to the factor
that could cause the disease. The two groups are examined over time to observe if the disease
manifests in either groups. The second study is a cohort study because among a population is
divided into physicians that smoked (exposed) to physicians that did not smoke (unexposed).
Question 3: Why might hospitals have been chosen as the setting for this study?
Hospitals have been chosen for the setting of this study because researchers are looking for individuals
that have already developed lung cancer; patients with this disease are more likely to found in a
healthcare setting.
Question 4: What other sources of cases and controls might have been used?
Other sources of cases could be physicians’ practices, clinics or cancer registries. Controls
could be selected from non-hospitalized patients living in the community.
Question 5: What are the advantages of selecting controls from the same hospitals as cases?
The advantage of selecting controls that are from the same hospital as the cases is that there
are not other factors/exposures different between the cases and the control that could lead to
the disease.
The advantages of using controls for patients from the same facility are: They are easier
to identify. They are more likely to participate than general population controls. They
minimize selection bias because they generally come from the same source population
(provided referral patterns are similar).
Question 6: How representative of all persons with lung cancer are hospitalized patients with lung
cancer?
The hospitalized patients with lung cancer does not take into account the people with lung
cancer who have not been hospitalized as yet or may not ever be hospitalized. Additionally, it
does not factor in the patients who have had lung cancer and have already died. By focusing
only on hospitalized patients there are only looking at cases of lung cancer that have
progressed to the point where they are hospitalized.
Question 7: How representative of the general population without lung cancer are hospitalized
patients without lung cancer?
The hospitalized patients are not very representative of the general population because they are
a group of individuals with other diseases/disorders. Comparably, the general population
considers of people with/without disorders and diseases
Question 8: How may these representativeness issues affect the interpretation of the study's
results?
The proportion of individuals who have the disease and have been exposed may not be representative of
the proportion of the general population that could manifest the disease if they have been exposed to the
exposure.
Question 9: From this table, calculate the proportion of cases and controls who smoked.
Proportion smoked, cases: 1350/1357=0.99
Proportion smoked, control: 1296/1357=0.96
Question 10: What do you infer from these proportions?
The proportion of cigarette smokers between cases and controls is similar, but larger in the cases group.
Question 11a: Calculate the odds of smoking among the cases.
Odds that a case was exposed = 1350/7
Question 11b: Calculate the odds of smoking among the controls.
Odds that a control was exposed = 1296/61
Question 12: Calculate the ratio of these odds. How does this compare with the cross-product
ratio?
Ratio of odds= (1350/7)/ (1296/61) = 192.85/ 21.26= 9.07
Cross-Product = (1350x61)/(1296x7)=9.08
Ratio of odds=Cross-Product
Question 13: What do you infer from the odds ratio about the relationship between smoking and
lung cancer?
The odds ratio is greater than 1, indicating that smoking is positively related to lung cancer. This suggests
that there is some association between smoking and lung cancer.
The smoker has nine times the odds of having lung cancer than a non-smoker.
Table 2. Most recent amount of cigarettes smoked daily before onset of the present illness, lung cancer
cases and matched controls with other diseases, Great Britain, 1948-1952.
Daily number of cigarettes
# Cases
# Controls
Odds Ratio
0
7
61
referent
1-14
565
706
6.97
15-24
445
408
9.48
25+
340
182
16.3
All smokers
1350
1,296
9.45
Total
1357
1,357
Question 14: Compute the odds ratio by category of daily cigarette consumption, comparing each
smoking category to nonsmokers.
Odds ratio by category of daily cigarette consumption: (cases/control)/(cases of referent/control of
referent)
1-14- (565/706)/ (7/61)= (.800)/(.115)=6.97
15-24- (445/408)/ (7/61)=(1.091)/(.115) = 9.48
25+(340/182)/ (7/61)=1.87/0.115=16.3
All smokers- (1350/1296)/ (7/61)= (1.04/0.11)=9.45
Question 15: Interpret these results.
Because all the odds ratios are greater than one, it is inferred that there is a clear association between
smoking and lung cancer. The highest association is amongst the 25+ daily number of cigarettes group,
and the lowest is in the 1-14 daily number of cigarettes group.
Question 16: Although the study demonstrates a clear association between smoking and lung cancer,
cause-and-effect is not the only explanation. What are the other possible explanations for the
apparent association?
The occurrence of lung cancer is also associated with
 indoor and outdoor air pollution,
 Secondhand smoke – like smoke from other person cig, pipes
 Family history of lung cancer
 Chest radiation therapy
 Diet habits, example- radon and arsenic in water
 Radon- Radon is a naturally occurring gas that forms in rocks, soil, and water. It cannot be
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seen, tasted, or smelled. When radon gets into homes or buildings through cracks or holes, it can
get trapped and build up in the air inside. People who live or work in these homes and buildings
breathe in high radon levels.
hazardous substances- found at some workplaces that increase risk
include asbestos, arsenic, diesel exhaust, and some forms
of silica and chromium.
alcohol drinking
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Air quality
Immunocompromisation
Smoking method
The use of other substances
Diet and lack of exercise
Occupation (mainly doctors were surveyed)
SES factors
Question 17: How might the response rate of 68% affect the study's results?
The results of the study may not be fully representative of the population, but it could still offer valid study
results, and gives a good idea of the correlation between smoking and lung cancer mortality in
physicians. Specifically, if only a certain demographic of the physicians responded. 40,637 (68%)
physicians, of whom 34,445 were males and 6,192 were females. Other occupations, and identities (race
and SES) were not accounted for.
Question 18: Compute lung cancer mortality rates, rate ratios, and rate differences for each
smoking category. What do each of these measures mean? (pg. 242 in Epi textbook)
Mortality rate: (death from lung cancer/person years at risk) x 1000
Rate Ratio: Mortality Rate / Referent Mortality Rate
Rate Difference: Mortality Rate - Referent Mortality Rate
What do each of these measures mean: The rate ratio shows how much more likely the exposed group
is to die from lung cancer in comparison to the unexposed group. The rate difference shows if the
mortality rate for the exposed group is greater than the mortality rate for the unexposed group and by how
much.
Question 24: What do these data imply for the practice of public health and preventive medicine?
The rate of mortality from lung cancer reduces as the amount of time that the person has quit increases.
This means that it is important to encourage people to quit smoking as it would reduce their risk for lung
cancer. The rate is lowest for nonsmokers, and this tells us that it is important to encourage people not to
start smoking at all! PREVENTION IS CURE!!!
As we can see in the Table.5, the number of lung cancer deaths occurred in ex-smokers who fall in the
category of years <5 and 5-9 years compared to smokers in more no of years. One of the reasons is that
during the early years after quitting, they may have severe side effects and damage due to smoking that
leads to more deaths even after quitting.
Question 25: Compare the results of the two studies. Comment on the similarities and differences
in the computed measures of association
Both the cohort and the case-control study show a positive correlation between smoking and lung cancer,
but the cohort study shows a stronger correlation than the case-control study.
Based on Table.6, if we compared the two study groups based on daily number of cig smoked, we can
clearly see a difference in the group 3 (15-24), 4 (25+) and All smokers. The rate ratio values were higher
in cohort study group compared to the case-control study.
The odds ratios in the case-control studies are consistently smaller than the rate ratios. An explanation for
this discrepancy is related to the use of hospitalized controls in the case-control study. As discussed
earlier, use of hospitalized controls was likely to result in a bias that tended to underestimate the risk
associated with smoking.
Question 26: What are the advantages and disadvantages of case-control vs. cohort studies? (pg
195 in epi textbook, chapter 9)
Case Control
Cohort
Sample Size
Allow for Smaller sample size
Large sample size
Costs
Less expensive
Expensive
Study Time
Shorter study time
Long study time
Rare Disease
Better suited to study when disease is rare
Better when disease is frequent
Rare Exposure
Harder with rare exposure
Better suited for rare exposure
Multiple
Exposures
Convenient to study multiple exposures
Can study multiple exposures
Multiple
Outcomes
Does not study multiple outcomes
Convenient to study multiple outcomes
Progression,
spectrum of
illness
Lower spectrum of illness
Greater spectrum of illness
Disease Rate
Odds ratio, Attributable risk
Absolute risk, Relative risk, Odds ratio
Recall Bias
High recall bias
Less prone Recall bias
Factors contributing to this bias
-age of the participants
-participants memory affected due to
alcohol od drug or any other similar factors.
-The length of time between when event
occurred and when it was reported.
But this bias is common in
retrospective cohort studies.
Loss to Follow
Up
High participant drop out
Loss to follow up bias
Selection Bias
High selection bias
Less prone to Selection bias,
it occurs in this study group if rate of
participation or the rate of loss to
follow-up differ by both exposure and
health-outcome status.
Selection bias is introduced in controls
when there is lack of correspondence
between source of cases and selected
controls with respect to calendar time,
health care seeking behavior and other
attributes.
The primary source of selection bias
are loss to follow up, withdrawal and
non-response.
Question 27: Which type of study (cohort or case-control) would you have done first? Why? Why
do a second study? Why do the other type of study?
The case-control study was quicker, easier, and less expensive. The cohort study was more difficult
and expensive to complete, and was slower to yield results. However, completion of the cohort
study provided confirmation of the findings from the case–control study and allowed for direct
calculation of rates and more intuitive interpretation of results.
Case control first because it is less expensive and has a shorter study time. After finding some incidence or evidence,
I'd then conduct a cohort study to verify and expand on the case control study.
Question 28: Which of the following criteria for causality are met by the evidence presented from
these two studies?
YES
Strong Association
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NO
Consistency among studies
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Exposure precedes disease
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Dose-response effect (relationship between amount of exposure and the
outcome)
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Biological plausibility
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