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Community Medicine: Lecture 6: Study Design _________________________Dr. Hassan Baey
Babylon University – College Of Medicine
Department of Community Medicine
Lectures in Community Medicine
For 4th Stage Students
By Dr. Hassan Baiee
2011 – 2012
Lecture 6
Study Design
Study Designs:
1. Observational studies:
a- Descriptive studies:1- Case report.
2- Case series.
3- Ecological studies (correlational)
4- Cross-sectional (prevalence studies)
b- Analytical studies:1- Case control studies (case references)
2- Cohort studies (prospective studies)
2. Experimental studies (intervention studies):
a- Randomized controlled trials (Clinical trials)
b- Field trials
c- Community trials (Community intervention studies)
Observational studies:Allow nature to take its course: the investigator measures but does not
intervene. They include descriptive or analytic studies.
A descriptive study is limited to a description of the occurrence of a disease
in a population and is often the first step in an epidemiological investigation.
An analytic study goes further by analyzing relationships between health
status and other variables.
Advantages of descriptive studies:
1. They use already available data.
2. They are less expensive and less time-consuming.
3. They describe the pattern of disease occurrence.
4. They formulate research hypothesis.
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Community Medicine: Lecture 6: Study Design _________________________Dr. Hassan Baey
Case reports:
Physician who sees unusual presentation of a common disease or a very
rare disease and he describes the findings of this case, case description may
take any relevant description of findings.
An unusual case may add to our knowledge, for example MI in very young
person without ECG finding is unusual presentation of MI.
Case series (Clinical series):
When a group of cases of the same disease are reported but in this
study we can not:
 Study the etiology of the disease
 Testing hypothesis
 Have a control group
But in this study it can help in generating hypothesis.
Example: vaginal ca is very rare in young females, if 10 cases were reported
and studied it may help us to generate a hypothesis when we interrogated
with stilbestrol taking during pregnancy among their mothers (this can be
done by further studies).
Discovery of AIDS was a case series.
Many clinical studies are case series.
Advantages of case reports or case series:
1. Use available clinical data.
2. Detailed individual data.
3. Add to our knowledge.
4. Suggest need for investigation (hypothesis generation).
Disadvantages of case report or case series
1. May reflect experience of one person or one clinician.
2. No explicit comparison group.
Ecological Studies (correlational):
based on studying of a group of people not individual as in previous studies.
Correlation data represent average exposure level rather than individual level
so we do not have each person information.
Ecological studies can generate a hypothesis and need further confirmation, it
provides just initial clues to causation.
Example: as meat consumption in different countries increases prevalence of
ca colon increases, Cigarette smoking increases ca lung prevalence increases
as well.
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Community Medicine: Lecture 6: Study Design _________________________Dr. Hassan Baey
Advantages of Correlational Study:
1. They describe the disease in the entire population in relation to the
factor of interest.
2. They use the correlation coefficient (r) to measure the association
between the two variables of interest.
3. They are easy to do, inexpensive and can be conducted quickly.
4. They represent the first step in searching for exposure-disease
relationship (generate hypothesis)
5. They use available data (administrative or other aggregate data).
Disadvantages of Correlational Study:
1. Correlation data represent average exposure level rather than actual
individual values. Data on exposure and data on outcome are collected
independently.
2. No assurance that persons with exposure (risk factor) of interest are the
same ones with outcome (disease) of interest.
3. Inability to link exposure with the disease in particular individual.
Cross-Sectional studies (prevalence studies or surveys):
It is a study of a group of people at a point in time for the prevalence of a
disease or an attribute in a well defined population but data is collected at
individual levels. In this study the measurements of exposure and effect are
made at the same time.
Survey either:
Population survey = census
Or
Sample survey
Advantages of cross sectional study:
1. Provide generalization from the sample to the population.
2. They are short term studies and not expensive.
3. Provide good information for the health problems and good for health
planners to assess health care needs.
4. Used for generating hypothesis to be test in further studies.
Disadvantages of cross sectional study:
1. It is difficult to separate cause and effect because measurement of
exposure and disease are made as one point of time and it is impossible
to determined which came first.
2. Cases detected are prevalent cases (survivors) leading to survival
bias cases cured or died are not detected.
3. Non response and this will affect the representation of the sample.
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Community Medicine: Lecture 6: Study Design _________________________Dr. Hassan Baey
Case Control Study:
A case control study begins with the selection of cases which should
represent all cases from a specified population. The most difficult task is to
select controls; the controls should represent people who would have been
designated study cases if they had developed the diseases. An important
aspect of this study is the determination of the start and duration of exposure,
the exposure status is usually determined after the development of the disease
(retrospective) and usually by direct questioning of the affected person.
Exposure may be determined by biochemical measurement, established
recording system.
Direction of inquiry
Time
Exposed
Disease
Non-exposed
Population
Exposed
No disease
Non-exposed
Figure shows case control study design
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Community Medicine: Lecture 6: Study Design _________________________Dr. Hassan Baey
Sources of selection of cases in case control study
 Hospital-based case control study: the cases will be identified from the
hospital or other health care facilities. These are common, relatively easy
and inexpensive.
 Population-based case control study: It involves locating and obtaining
data from all affected individuals or a random sample from population.
Selection of controls in case control study:
It is the most difficult aspect of Case Control Study (CCS), it depends on:
1. Characteristics and sources of cases.
2. Need to obtain comparable and reliable information from cases and
controls.
3. Practical and economic considerations.
The control should be comparable to the source of the population of cases.
Any exclusion or restrictions made in the selection of cases should be applied
equally to the controls and vice versa.
Advantages of case control studies:
1. Suitable for rare diseases.
2. Results can be obtained quickly.
3. Relatively inexpensive and short term study.
Disadvantages of case control studies:
1. Incidence or absolute risk cannot be determined directly from a case
control study.
2. Difficulty in selection of the control.
3. Case-control studies rely upon retrospective data which lead to recall
bias.
4. Because the data are collected after the event (retrospectively) it is
difficult to be whether correlation is causal or not.
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Community Medicine: Lecture 6: Study Design _________________________Dr. Hassan Baey
Example of case control study of coronary heart disease (CHD) and cigarette
smoking:
CHD
Smoke Cigarettes
Do not smoke
a
Control
b
112
c
176
d
88
224
Total
200
400
% of smoking
56%
44%
odd’s that cases exposed
Odd’s ratio = ‫ـــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ‬
odd’s that control exposed
cases with exposure × control without exposure
= ‫ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ‬
control with exposure × cases without exposure
ad
a/c
= ‫ـــــــــــــــ = ـــــــــــــ‬
bc
b/d
112 × 224
25088
= ‫ = ــــــــــــــــــــ = ـــــــــــــــــــــــ‬1.6
176 × 88
15488
The exposure is positively related to the disease because the odd’s ratio is
greater than 1
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Community Medicine: Lecture 6: Study Design _________________________Dr. Hassan Baey
Cohort Study:
Follow-up study or incidence study.
Begin with a group of people (a cohort) free of disease, who are classified
into subgroups according to exposure to potential cause of disease, and the
whole cohort is followed up to see how the subsequent development of new
cases of the disease differ between the groups with and without exposure.
Cohort study is a longitudinal study. Cohort study provides the best
information about the causation of disease and the most direct measurement
of the risk of developing disease.
Direction of inquiry
Time
Diseased
Exposed
Non-diseased
Population
Cohort
Diseased
Not
exposed
Non-diseased
Figure shows cohort study design
Advantages of Cohort studies:
1. They allow complete description of the individuals experience
subsequent to exposure.
2. They provide clear temporal sequence of exposure and disease.
3. They provide excellent opportunity to study rare exposures.
4. They permit the assessment of multiple outcomes that may be related to
the same risk factor.
5. They permit the direct assessment of risk of exposure and disease
development .
6. Less chance of bias .
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Community Medicine: Lecture 6: Study Design _________________________Dr. Hassan Baey
Disadvantages of Cohort studies:
1. Not suitable for rare disease.
2. Long-term follow up may be needed when the latency period for
outcome of interest is long.
3. Attrition or loss of people from the sample during the course of the
study.
4. Time consuming.
5. Expensive.
6. The exposure status may change during the conduct of the study.
Experimental Studies (intervention):
The investigator exercises control over the independent factors manipulating
them to see the effectiveness of the new drugs or preventing methods for a
certain disease .
The investigator would intervene, given the drug to one group of patients but
not to another, properly conducted experiment is the most powerful way of
establishing cause and effect relationship.
This method in medical research commonly takes the form of randomized
control trials.Which study a new preventive or regime.
Randomized control trial (RCL) is an epidemiological experiment to study a
new preventive or therapeutic regime. Subjects in a population are randomly
allocated to groups, usually called treatment and control groups and the
results are assessed by comparing the outcome in the two or more groups.
The outcome of interest will vary but may be the development of new disease
or recovery from established disease. To ensure that the groups being
compared are equivalent patients are allocated to them randomly i.e. by
chance. Within the limits of chance randomization ensures that control and
treatment groups will be comparable at the start of an investigation. Any
differences between groups are chance occurrence unaffected by the
conscious and unconscious biases of the investigators.
The investigation under test may be a new drug or a new regime, such as
early mobilization after myocardial infarction.
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Community Medicine: Lecture 6: Study Design _________________________Dr. Hassan Baey
Problems of interventional studies
a. Ethical: there should be always some doubt in the beneficial or harmful
effect of an agent procedure to be tested.
b. Feasibility: it is not always feasible.
c. Cost: the cost is higher than other studies.
Blindness:
a) Single blindness:
The patients are not aware about the drugs they received.
b) Double blindness:
Neither the investigator nor the patients are aware about the drugs
received by the study group or the control group.
Study Population
Selection by
defined criteria
Potential
Participants
Nonparticipants
(do not meet
selection criteria)
Invitation to
participate
Participants
Nonparticipants
Randomization
Treatment
Control
Design of randomized controlled trial (RCT)
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Community Medicine: Lecture 6: Study Design _________________________Dr. Hassan Baey
Myocardial Patients (507)
Uncomplicated
(179)
Randomized
(80)
Complicated.
Excluded
(328)
Not included in study
(99)
Early
discharge
(40)
Late discharge
(40)
0
6
0
3
0
10
5
8
Outcome:
Deaths
Hospital re-admissions
Re-infarctions
Patients with angina
Randomized controlled trial of early hospital discharge after
Myocardial infarction
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