Epidemiological study designs by anil mph

Epidemiological Study Designs:
• Broadly, the epidemiological studies may be
classified in to the following two broad
categories, based on the types of exposure:
1. Observational Study
2. Experimental Study
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By: Anil Chaudhary (MPH), NOC, Lalitpur,
Nepal.

According to WHO classification:
Observational studies Experimental studies
Descriptive study Clinical trial (RCT)
Analytical study Field trial
Cross sectional study Community trial
Ecological study
Case control study
Cohort study
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1. Observational studies: in observational study, the researcher
has no control over the exposure in another word , the
allocation or assignment of factors is not under the control of
investigator; he (she) just observe the situation/event and
records it, which may be descriptive or analytic. The
observational study is also called non-interventional study.
2. Experimental studies: In experimental study, the researcher
has control over the exposure, he doesn’t only observe the
situation and record it, but also manipulate the situation and
measure the effect of manipulation. The experimental study
is also known as interventional study.
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Observational studies
Descriptive
- Case report
- Case series
- Ecological
- D. cross-sectional
Analytical
- Cohort
- Case control
- Analytical Cross sectional
- A. Ecological
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Observational Study Designs
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Descriptive Studies
 Studies that focus on characterizing morbidity or mortality in
populations by person, place or time variables.
 They do not have a priori hypothesis.
 Are not designed to test preconceived suppositions about the
relationships between exposure and outcome.
 Focus is on describing what exists.
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Descriptive study types:
1. Case reports
- detailed description of an individual(patient)
2. Case series
- Describes characteristics of a group or cluster of individuals
with the same disease or symptoms.
- Presents a relatively complete profile of case (disease,
illness)
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Cont…
3. Ecological Study
- The unit of analysis is group such as countries, states, cities,
hospitals or schools
- Investigator have a summary measure of exposure and a
summary measure of outcome for each group being studied
- Data on exposure and outcome for individual is not
collected.
- Association is identified only at group level.
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Cont…
• If we draw conclusion for individual based on
ecological study findings we commit an
ecological fallacy
• Ecological fallacy is an error of reasoning that
one commits when group associations are
used to draw conclusions about associations
at the individual level that may not
necessarily exist.
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Types of ecological studies
Exploratory studies
• Descriptive studies that look at variations in
outcomes across ecological units such as
countries, states etc.
• Exposure is not assessed
Multiple- group comparison studies
• Looks for or test associations between average
exposure levels and overall outcome rates.
• Generally based on secondary sources
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Cont…
Time – trend studies
Detect changes in the average exposure level and outcome rates
for a single population over time
Mixed studies
Examines changes in average exposure level and outcome rates
for several population over time.
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Cont…
4. Cross- sectional study
• One observes a sample of a population at a single point or
brief period of time.
• Exposure and outcome assessed at same time period.
• The unit of analysis is individual
• Also referred as prevalence studies.
• Prevalence rates are the usual measure employed in the
analysis stages.
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Cont…
a b
c d
Present (Dis.) Absent (Dis.)
Outcome Status
Exposure Status
Exposed
Unexposed
a+ b
c+ d
a + c b+ d n (a+b+c+d)
Crude Prevelance
rate (PR) =
{(a+c)/n} * 100
PRe =
[a/(a+b)]*100
PRue =
[c/(c+d)]*100
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Analytical studies
• Analytical studies are designed to test hypothesis about the
association between an exposure of interest and a particular
outcome.
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Case control studies
• Begins by classifying subjects according to their outcome
status.
• Cases of the disease being investigated are selected along
with a comparable group of control who do not have the
outcome in question.
• The cases and controls are then examined for exposure of
interest.
• In case control study one is going from effect to cause.
• Cases and controls are selected and data are collected about
past exposure that may have contributed to disease.
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Cont…
 Seeks to identify possible cause of the disease by finding out
how the cases and controls differ.
 Takes into consideration that the case groups must have
been exposed to some factors that contributes to the
causation of their disease.
 Compares the frequency of exposure among cases and
controls
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Schematic Diagram of Case Control Study Design
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Selection of cases
 First step: Establishing the diagnostic criteria and definition
of disease.
 Incidence (new cases) - preferred
 Prevalence cases are more difficult to link a specific cause
with a disease outcome – because it is influenced by the
development and duration of disease.
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Selection of cases
• Sources of cases:
– Records from public health clinics
– Physician offices
– Health maintenance organizations
– Hospitals
– Industrial and government sources.
• Cases should be representative of all persons with the
disease.
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Selection of cases
• All persons with the disease may be included in the study.
• However, random selection is required.
• A large random sample from a population of interest ensures
the results can be appropriately generalized.
• Researchers can use restriction (limiting subjects to certain
characteristics) to reduce potential bias and to increase
feasibility.
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Selection of controls
• In order to ensure a valid and reliable case-control study, the
controls should look like cases with the exception of not
having the disease.
• Controls should be selected from the same population as the
cases.
• Controls should be representative of general population.
• Controls should have the same probability of being selected
and exposed as the cases.
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Selection of controls
• Sources of controls:
– General population as cases
– Same hospital as cases
– Family, friends, or relatives of cases
• In some circumstances, selecting more than one control
group may be useful.
• It may also be useful to select more controls when few cases
are available. This increases power of the study
• The ratio of controls : cases should not exceed 4 : 1.
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Exposure Status
• Information about exposure status can be obtained through
– Medical records
– Interviews
– Questionnaires
– Surrogates (such as spouses, siblings, or employers)
• Information on exposure status between cases and controls
should be collected in a similar manner (in order to avoid
bias).
• Blinding interviewers or those assessing medical records to
cases and controls may further minimize bias.By: Anil Chaudhary (MPH), NOC, Lalitpur,
Nepal.
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Exposure Status
• Preferable to blind those performing the assessment
to the hypothesis of the study (because such
knowledge could influence how they probe or scan
records for information).
Nepal.
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Measure of association
• Commonly used measure of the relative
probabilities of disease in case-control studies is
ODDS RATIO (relative odds).
• ODDS RATIO: is appropriate for measuring the
strength of association between exposure and
disease variables.
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ODDS RATIO
Cases Controls Total
Exposed a b a + b
Unexposed c d c + d
Total a + c b + d N = a + b + c + d
Odds of exposure among case group = [a/(a+c)]÷ [c/(a+c)] = a/c
Odds of exposure among control group = b/d
Odds Ratio (OR) = (a/c) ÷ (b/d) or ad/bc
Odds of an event can be defined as the ratio of the proportion of cases
exposed to proportion of cases not exposedBy: Anil Chaudhary (MPH), NOC, Lalitpur,
Nepal.
25

ODDS RATIO (OR)
• If the OR = 1, this indicates no association between exposure
and disease.
• If the OR > 1, this indicates a positive association between
exposure and disease.
• If the OR < 1, this indicates a negative association between
exposure and disease.
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Cohort study
• A cohort is defined as a population group, or subset thereof (
distinguished by a common characteristics), that is followed
over a period of time.
• Best method for determining the incidence and natural history
of a condition.
• Because data on exposure and disease refer to different points
in time, cohort studies are longitudinal in nature.
• In single cohort studies, those who do not develop the disease
are used as internal controls.
• When two cohorts are used, one group has been exposed or
treated with an agent; and the other group not treated with
agent act as external controls.
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Cont…
• Start with a population at risk
• Measure characteristics at baseline
• Follow-up the population over time with surveillance or re-
examination
• Compare events rates in people with and without
characteristics of interest
– Study participants may be either fixed (subjects do not vary
over time and dropouts are not replaced) or dynamic (where
new subjects enter the study in accordance with eligibility
criteria.
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Schematic Diagram of Cohort Study Design
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Prospective cohort studies (90%)
• The term prospective refers to the timing of data collection
and not to the relationship between exposure and effect.
• In this type of study, a group of people are chosen who do
not have outcome of interest on the basis of exposure and
followed for a long time until the events occur.
• The cohort are then followed longitudinally into the future
to determine if the rate of development of the study outcome
is significantly higher in the exposed group compared to
unexposed group.
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Retrospective Cohort (10%)
• Makes us of historical data to determine exposure level at
some base line in the past
• Follow up for subsequent occurrence of disease between
baseline and the present is performed.
• The cohort is considered historical as it represents a group
of people once they were in the past.
• Once cohort has been identified, next step is to classify
subjects by exposure status.
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Measure
Disease No disease Total
Exposed a b a + b
Unexposed c d c + d
Total a + c b + d N = a + b + c + d
Relative Risk = Incidence rate in the exposed / Incidence rate in the non exposed
= [a/(a+b)] ÷ [c/(c+d)]
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Cont…
• RR = 1 , implies that the risk(rate) of disease
among the exposed is no different from the risk of
disease among the non exposed
• RR>1, implies risk is high among exposed
• RR<1, Implies exposure is not fully associated with
risk of disease
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Experimental Study Designs
1. Randomized Control Trials
2. Non Randomized Trials
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Randomized Control Trial
• Is an experiment where individual subjects are
randomly assigned to either an experiment or
control group to assess the effects of a preventive or
therapeutic agent, treatment, procedure, or service.
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RCT:Can be grouped into 3 categories
1. Preventive Trials
2. Therapeutic trials
3. Intervention trials
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Design and analysis of a RCT may be considered
in ten stages:
1. Specification of the program
2. Subject selection
3. The observation
4. Allocation of subject
5. The control group
6. Required no of subjects
7. Control of subjective factors
8. Follow up
9. Outcome
10. analysis
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Process
1. Develop the hypothesis
2. Selection of sample
First, Reference population must be specified
Next, identify experimental population
Finally, sample population is determined.
Eligibility criteria( exclusion and inclusion)
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3. Allocate subjects into Experimental
and Control groups
Randomization
The subjects have same probability of being
assigned to both group
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4. Apply the intervention
- Experimental group exposed to promising but
unproven agent, treatment, procedure or
service
- Control group exposed to either a placebo or
the customary treatment for the problem
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5 . Assess the outcome during follow
up
- Subjects are followed up prospectively
- Assessment of outcomes during follow up
may be subject to measurement bias
- Bias can occur when subjects are aware of
which group they have been assigned to
- Placebo effect can occur
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Bias can be minimized through blinding
(masking)
- Single blinding: A single blind experiment only the participant
does not know whether they are part of the treatment or
control group,
- Double blinding: A double-blind experiment both the
participant and the experimenter are kept in the dark as to
who is in the treatment and who is in the control group
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Randomized Control Trial (RCT):
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Other designs of randomized control trials
1. Restricted randomization :
 randomization and comparison is done within strata
2. Sequential analysis :
no. of observation not fixed in advance
Data continuously analyzed
Trial interrupted as soon as a statistical significance is
reached
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Cont…
3. Factorial design
• Simultaneous evaluation of two or more treatment
as well as their outcomes
4. Trials on collective units
• Random distribution of group of individuals
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Difficulties in application of RCT for
evaluation
- Randomization is not always possible (ethical issues )
- Randomization does not answer the crucial questions,
regarding what is the actual impact
- Comparability resulting from randomization is frequently
partly destroyed (double blinding rarely applicable)
- The randomized comparison of individual components
impossible as evaluation is multidimensional
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Non randomized trials
• A study where participants have been assigned to the
treatment, procedure, or intervention alternatives by a
method that is not random. The investigator defines and
manages the alternatives
• It do not meet the criteria for a true experimental design
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When is it appropriate to use a non-randomized trial
design?
• When the act of random allocation may reduce the
effectiveness of the intervention
• When it would be unethical to do random allocation
• When it is impractical to do random allocation
 (e.g. cost or convenience factors)
• When there are legal or political obstacles to random
allocation
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Various types
- Uncontrolled trials with a single examination after the
intervention
- Before and after comparison
- Controlled non randomized trials
- Non randomized controlled trials based on collective units
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Epidemiological study designs by anil mph

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