Epidemiological study designs by anil mph

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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 Study2. Experimental Study

By: Anil Chaudhary (MPH), NOC, Lalitpur, Nepal.


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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

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.


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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).


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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 RATIOCases 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.

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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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MeasureDisease 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 Trials2. 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 trials3. Intervention trials


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Design and analysis of a RCT may be considered in ten stages:

1. Specification of the program2. Subject selection 3. The observation4. Allocation of subject5. The control group6. Required no of subjects7. Control of subjective factors8. Follow up9. Outcome10. analysis


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Process1. Develop the hypothesis 2. Selection of sample

First, Reference population must be specifiedNext, 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

RandomizationThe 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 advanceData 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

50By: Anil Chaudhary (MPH), NOC, Lalitpur, Nepal.

Thank You

Epidemiological study designs by anil mph

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