epidemiological studies, descriptive 2011 colour less 2 slide per page non-coloured

8/3/2019 EPIDEMIOLOGICAL STUDIES, Descriptive 2011 Colour Less 2 Slide Per Page Non-coloured

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Epidemiology Studies,Descriptive designs

Addis Ababa University

School of Public Health

Negussie Deyessa, MD, MPH, PHD

2011

Negussie D, 2011

Learning Objectives When you have completed this session you will

be able to:1. Describe well all types of epidemiological study designs

2. Explain the uses of the various descriptive study designs.

3. Illustrate well the characteristics of descriptive studydesigns and depict how hypothesis is generated.

4. Determine when to proceed with an analytic study tofurther test a hypothesis

5. Describe the characteristics and design of analytic(observational) studies

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Strategy of epidemiology

Epidemiologic surveillance

Epidemiologic research

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Purpose of Epidemiology1. To diagnosis community health problems

2. To identify natural history and etiology of

diseases.

3. To evaluate and plan health services.

To achieve these purposes, it is crucial to

understand basic types of

epidemiological study designs.

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Definition

The study of the frequency, distribution anddeterminantsof diseases and other health relatedproblems in human population and the application ofthis to the prevention and control of health problems.

o Frequency: Quantification of the existence or occurrenceof disease

o Distribution: Describes the events of disease in terms ofperson, place and time.

o Determinants: deals with factors affecting the distributionor occurrence of disease or health problem.

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Scope of epidemiology Measuring the frequency of diseases and events

(mortality, births, morbidity incidence andprevalence, hospital stays, drug use, injuries,health behaviors..);

Solving epidemics;

Looking for causes of disease and risk factors;

Evaluating new diagnostic tests;

Evaluating new treatments;6


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Cont

Surveillance for new diseases and changes in oldones;

Searching health services, their availability andtheir problem;

Costing out alternative diagnostics, treatments orhealth service provisions.

Thus, it is crucial to understand basic types of

epidemiological study designs.

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Why Epidemiological Studies? To answer questions like:

How big is the problem (magnitude)? Prevalence, incidence, mortality

What, who and where of any health problem? Person characteristic of affected population Place characteristics (locality)

What factors are associated with certaindisease

Specific factors related to causation

To evaluate interventions Which drug is best for patients with X disease To evaluate any program

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What is a research?

It is a systematic approach to problem

solving

It Investigates a phenomenon to answer a

burning question

It tries to answer a vague study question

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Assumptions for scientificresearch methods

Objective reality exists independent from

peoples perception

Nature has order, regularity and consistency

All phenomena have causes that can be

discovered

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Limitations of research based on

scientific methods No single study proves or disproves a

hypothesis

Ethical issues can constrain researchers

Adequate control is hard to maintain in a

study

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Categorizing ResearchTwo forms of approach

1. Qualitative Vs Quantitative (Words Vs Numbers)

Qualitative research Data from words, pictures, gestures etc

(more on social and psychological phenomena)

Quantitative research Data from numbers,

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Cont

2. Basic Vs Applied research

Basic R:

Undertaken to advance knowledge in a given area

understanding relationships among phenomena

(Research done within laboratories)

Applied R:

Undertaken to remedy a particular problem or modify

a situation, to make decisions or evaluate techniques

(Researches made within human beings behavior)

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Major difference

Basic research

Goal:

to understand and explain factsabout a phenomena

Discipline specific

Contribution:

a theory to explain the phenomenaunder investigation

Eg

How did HIV replicate within ahuman cell?

Applied Research

Goal:

to understand societal problemsand identify potential solution

Takes an explanation to real-world problems

Inter disciplinary

Contribution:

solutions to real world problems

Eg

How can epidemiologists controlthe spread of HIV/ AIDS?

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Categories of epidemiologicalstudies

1. Descriptive epidemiological studies

Population as study subject

o Correlational /ecological studies

Individual as study subjects

o Case report / Case series

o Cross-sectional surveys

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Cont

2. Analytic epidemiological studies

2.1 Observational studies

o Case-control study

o Cohort study

2.2 Experimental / intervention studies

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Case-control

Cohort

Individuals

Intervention

Retrospective

Prospective

Descriptive

Populations

Analytical

Observational

Case-series

Cross-sectional

Ecologic

Clinical trials

Epidemiological studies

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Descriptive versus Analyticalepidemiology

Descriptive epidemiology:

Generates idea(s)/ hypothesis to be tested using

analytic study design

Analytical epidemiology:

Uses comparison groups to establish an association

between risk factors and illness in the two groups

Tests a hypothesis18


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1. Descriptive Studies

Some studies simply describe occurrence of disease orhealth related problems

Prevalence of a disease,

Rate of certain behaviour

Describing these factors does not link any thing

However we can identify unusual distributions orcorrelations (e.g clusters)

These insights can be used to generate interestinghypothesis

(Case series, cross-sectional, ecological)

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We use descriptive studies forhypothesis formulation

Person

Who is getting the disease? Age, race, sex

Place

Where are the rates of disease highest/ lowest?

TimeWhen does the disease occur commonly/ rarely?

Is the frequency of the disease now different from

the corresponding frequency in the past?

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Descriptive design are useful for hypothesis

generation

Hypothesis is thought when observed

exposure among cases is higher/ lower than

expected;

It is usually generated based on knowledge,

experience or specific information we have.

Cont.

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What is a Hypothesis?

In epidemiology, a hypothesis is:

a supposition, that comes from observation or

reflection, that leads to refutable prediction

any assumption in a form that will allow to betested and refuted

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But sometimes also: Why?

What is the source of infection for an outbreak ofdiarrhoeal disease?

What are the risk factors for neonatal tetanus?

What factors are associated with increased mortality forpersons with measles?

Does smoking cause lung cancer?

Analytical epidemiology23

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Purpose/ Aim

1. To test hypothesis about causal relationship Proof Vs Sufficient evidence

2. To search for cause and effect.Why?? How??

3. To compare treatment regimens / prevention programs

4. To assess diagnostic tests

5. To quantify the association between exposure andoutcome

Measure of association

2. Analytic epidemiological studies

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Cont

It focuses on determinants of disease by testing

hypothesis.

Try to answer questions like why and how of a

disease.

Hypothesis is tested using explicit type of

comparison using appropriate comparison group.

Two study designs,1. Observational

2. Interventional designs.

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Cont Difference lies in the role of the investigator.

In Observational studies, the investigator simply

observes the natural course of event

In interventional studies, the investigator assigns

study subjects to exposure and non-exposure

then simply follows to measure for diseaseoccurrence.

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2.1 Observational studies

Information is obtained by simple observation of theevent.

Two basic types:a. Case control study:

b. Cohort study design

Major difference is in the method they initiate acomparison group

Comparison of groups is made either by differencein disease occurrence (Cohort studies) ordifference in exposure status (Case control studies)

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a. Case control study Cases (subjects having a specific disease) and

controls (subjects not having the disease) arecompared for their exposure status.

Cases are first selected then controls are similarlyselected and analysis is made on, to observeamong whom the exposure status is higher

It assess retrospectively on exposure status

It is relatively cheaper, (Time and Cost)

Measure of association is using Odds ratio28


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Design of case control

Exposed

Non-exposed

Exposed

Non-exposed

Cases

(People with

disease)

Controls

(People without

disease)

Population

TimeDirection of inquiry

Starting of Observation29

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b. Cohort study

Healthy subjects are classified on the basis of theirspecific exposure status and are followed up for aspecific time to determine for the development of anew disease.

Comparison between groups is made on differencein occurrence of a new disease between the twogroups

There is usually a follow up.

Relatively expensive (time, cost).

Measure of association is using Relative risk

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Population

People

with

out a

disease

Exposed

Not -Exposed

Disease

Disease

No disease

No disease

Direction of inquiry

Time

Cohort study design

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2.2 Interventional/ Experimental

o Investigator assigns subjects to exposure and non-exposureand makes follow up to measure for the occurrence of adisease.

o It is usually prospective.

o Very expensive,

o Difficult to overcome ethical issue.

o Measure of association is using Relative risk

o Three types

o Randomized controlled (clinical) trial (patients, treatment)

o Field trial (healthy people, prevention)

o Community trial (intervention at community level)

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Population

Patients

with a

disease

Experimental

group

Nonexperimentalgroup

Recover

Recover

Not recovering

Not recovering


Time

Experimental study design (Clinical trial)

Manipulation by investigator

Selection of people to be exposed or not-exposed34


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Population

People

with

out a

disease

Intervention

No-intervention

Disease

Disease

No disease

No disease


Time

Experimental study design (field trial)

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Selection of people to be exposed or not-exposed

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PopulationCommunity

with out a

disease

Intervention

No-intervention

Disease

Disease

No disease

No disease


Time

Experimental study design (community intervention trial)

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Selection of people to be exposed or not-exposed


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Temporal relationship of exposure and disease, andtime of assessment of different study designs.

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2. Descriptive epidemiology

Objectives

To evaluate trends in health and disease andallow comparisons among countries orsubgroups within countries

To evaluate a basis for planning, provision andevaluation of services

To identify problems to be studied by analyticmethods and generate a hypothesis related tothose problems

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Cont.

Describes the general characteristics of thedistribution of a disease in relation to person,place and time.

Who? Where? When?

It provides valuable information

To allocate resources efficiently and

To plan effective prevention or educationprograms.

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Cont

It provides the first important cluesabout possible determinants of adisease (formulation of hypothesis).

Hypothesis is formulated on an implicit

comparison ie comparison with theexpectation or experience.

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1. Method of difference:If the occurrence of a disease markedly differ then there isparticular factor.

2. Method of agreement:Observation of a single factor common to a number ofcircumstances.

Eg any diease and its symptoms

3. Method of concomitant variation:Circumstances in which the frequency varies in proportionto frequency of disease. More on Correlational studies

Hypothesis formulation

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Person

Place

Time

Cases

Descriptive Epidemiology

Who? Where? When?

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Descriptive studies

Population as study subject

o Correlational /ecological studies

Individual as study subjects

o Case report / Case series

o Cross-sectional surveys

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1. Correlational/ Ecological study

Uses data from entire population (as a whole) to

compare disease frequencies.

(ie it doesnt need data from individuals)

Can be done quickly and inexpensively, often

using already available data.

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Rationale for ecological studies

1. Low cost and convenient

2. Measurement limitation (difficult to measure at

individual level)

(eg environmental contact, dietary exposure)

3. Other designs may be unable to measure

4. Interest on ecologic effect

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Levels of measure

Source of data could be1. Aggregate measures:

(mean, proportion) observations derived from individualsin a group. (eg proportion of smokers, mean familyincome)

2. Environmental measures:Physical characteristics of a place in which each groupcommunities live or work (air-pollution, fluoride contentof water)

3. Global measure:Attributes of groups, organizations or places, for whichthere is no measure at individual level,(eg population density, presence of specific law)

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Level of analysis

Completely ecologic analysis; all variables areecologic measures and analysis is in a group.

Partially ecologic analysis; addition of someindividual variables and ecologic variables

Measures of analysis in Correlational studies isusing correlation coefficient (r)

Correlation coefficient (r) is a descriptivemeasure between continuous variables thatvaries between -1 and +1)

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Cont.

Fig. Factious data to show correlation between saltsold and mean diastolic BP. (positive r ~ 0.67)

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X

Y

X

r ~ +1

Y

Xr ~ -1

Y

r ~ 0 r ~ 0 X

Y

Correlation coefficient

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Levels of inference

It depends on the goal of inference wanted:

It can be to make biologic (bio-behavioral) inference

about individual risks, or (such inferences are usually

vulnerable to bias called Ecological fallacy)

It can be ecological inferences about effects on group

rates.

It is also possible to infer to contextual effect that may

be similar to biological effect.

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Limitations

Unable to link an exposure to occurrence ofdisease in a single individual.

Lack of the ability to control for effect ofconfounders.

Data represent average exposure levelsrather than actual individual values,

ecological fallacy or bias.

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2. Case reports or case series

Useful for the recognition of newdiseases,

Useful for constructing of the naturalhistory of a disease,

Use to formulate a hypothesis and todetect an epidemic

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A. Case report:

It is the study of health profile of a single

individual using a careful and detailed

report by one or more clinicians.

It is common form that is published inarticles

It is made using Simple history, Physical examination and

Lab. / radiologic investigation.53

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ContReport is usually documented if there is unusual

medical occurrence, thus it may be first clue for

identification of a new disease.

It is useful in constructing a natural history of

individual disease.

It was a single case report that formulated the

hypothesis of oral contraceptive use increases

venous thrombo-embolism.

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Individual case report can be expanded to a case

series, which describes characteristics of a number

of patients (usually 5-12) with a similar disease.

Similar to case report, it is usually made on cases

having new and/ or unusual disease (giving interest

to clinicians)

It is often used to detect the emergence of newdisease or an epidemics.

Eg. The first five AIDS cases in USA.

B. Case series

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ContExample:Five young, previously health homosexual menwere diagnosed as having Pneumocystis cariniipneumonia at Los Angeles hospital during a sixmonth period from 1980 to 1981.

This form of pneumonia had been seen almostexclusively among older men and women whose

immune systems were suppressed.

This unusual circumstance suggested that theseindividuals were actually suffering with a previouslyunknown disease, subsequently it was called AIDS.

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Cont

Both case report and case series are able toformulate a hypothesis but are not able to test forpresence of valid association.

Fundamental limitation of case report is presence

of a risk factor could be simply coincidental.

You are not able to test for association because

there is no relevant comparison group

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3. Cross-sectional surveys

Is generally called study of prevalence

Survey is conducted in a population, to find

prevalence of a disease and exposure.

Exposure and disease status are assessedsimultaneously among individuals at the

same point in time .

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Cont.

Cross-sectional surveys could provideinformation about the frequency of a diseaseby furnishing a snapshot at a specified time.

May be used first step in longitudinal or casecontrol studies.

Data are obtained Only once.

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Cont.The study may stay for more than 10 years with

continuous enrolment, and remains cross-sectional if data are obtained only once

Measures of association is made using oddsratio.

Has great value to public health providers to:

Assess the health status and health care needs of a

population

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Cont

It can be considered as analytic study, if it

assesses presence of an association.

For factors that remain unaltered overtime,

such as sex, race, blood group,

it can provide a good evidence.

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Increased ByBy longer duration of the

disease

Prolongation of life of patients

without cure

Increase in new cases

(increase in incidence)

In-migration of cases

Out-migration of healthy people

In-migration of susceptible

people

Improved diagnostic facilities

(better reporting)

Decreased ByShorter duration of the

disease

High case fatality

Decrease in new cases

(decrease in incidence)

In-migration of health people

Out-migration of cases

Out-migration of susceptible

people

Improved cure rate of cases)

Factors influencing Prevalence

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Limitations

Since exposure and disease status is

assessed at a single point in time,

temporal relationship between

exposure and disease can not be

clearly determined.

Egg and hen phenomena

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Advantage and disadvantage

Advantages

May study severaloutcomes

Controls over selection ofsubjects

Relatively short duration

Good first step for cohortstudy

Yields prevalence

Disadvantages

Doesnt establish sequenceof events,

(temporal relationship)

Potential bias in measuringexposure

Potential survivor bias

Not feasible for rare diseases

Doesnt yield incidences ortrue relative risk

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Learning Objectives

When you have completed this session you will beable to:1. Describe well all types of epidemiological study designs

2. Explain the uses of the various descriptive study types.

3. Express well the characteristics of descriptive study designsand how hypothesis is generated.

4. Determine when to proceed with an analytic study to further

test the hypothesis

5. Describe the characteristics and design of analytic(observational) studies

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Cohort study

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Dec, 2011


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Learning objectives

1. Describe the characteristics of a cohort study.

2. Describe the steps and application of cohort study design

3. List the conditions under which a cohort study is anappropriate choice to address a research question.

4. List the types of bias most likely to affect a cohort study.

5. Describe the advantages and disadvantages of a cohortstudy design.

6. Calculate appropriate measures of disease occurrence andexposure-disease association for a cohort study.

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Cohort study It is also called follow up, longitudinal,

prospective study.

The word cohort is used to designate a group ofpeople who share a common experience.

a Birth cohort,

a Cohort of smokers,

a Cohort of MPH graduates in 2009, etc.

It is an observational study that measuresincidence of disease occurrence.

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Cohort studies An investigator studies a group of exposed

and unexposed subjects and follows the

study subjects over a period of time and

compares the incidence of developing

disease of interest in the 2 groups

Exposed

Unexposed

Diseased

No disease

Diseased

No disease

PRESENT TIME FUTURE TIME

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1. Basic elements

Disease free at entry

Selected by exposure status rather thanoutcome

Follow up is needed to determine theincidence of the outcome in each exposuregroup For non communicable chronic diseases this may take

years

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a. Exposure status

Study subjects should be disease free

Define study subjects using inclusion and

exclusion criteria on the basis of exposure

status

Environmental factors: smoking, air pollution,

pesticides

Criteria can be specified by amount of

exposure

Eg. Cigarette Smocking (# of cigarette per 71

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b. Possible outcomes in cohort

No disease

Disease

Lost to follow up

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Cohort studies

Selected exposedand unexposed

Exp+

Exp -

Incidentcase

Incidentcase

Non-case

Non-case

Ascertaincaseness

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2. Features of cohortstudy

1. It shows temporal sequence

2. Good to assess effect ofrare exposures

3. Could assess multiple effects of a singleexposure

Exposure

Disease

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3. Types of cohort studies

Two forms of cohort study

The major difference is on the basis of

Initiation of study and theoccurrence of disease.

The two forms are similar, becauseselection of study subjects is made on the

basis of theirexposure status.75

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Cont

1. Classical (Prospective) cohort The exposure may or may not have

occurred at the time when the study begin,

but the outcome has certainly not yetoccurred.

2. Historical (Retrospective) Cohort Both the exposure and outcome have

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Cont.

1. Prospective cohort

Exposure status determined

at present

Study groups followed up and

disease outcome will be

ascertained in the future

2. Retrospective cohort

Exposure determined in the

past from records

Disease outcome ascertained

at present

Diseaseoutcome

Diseaseoutcome

Exposure

Exposure

Present FuturePast

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Cohort study designClassical (prospective)

1. measure exposure 2. measure outcome

Historical (retrospective)

2. measure outcome1. Record of exposure

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Exposure

?

1. Case controlDisease

?

Exposure, (+) or (-)? To be determined

Timing of case-control, prospective and retrospectivecohort study in relation to exposure and outcome.

Exposure?

?

2. Prospective CohortDisease

3. Retrospective Cohort

??

Exposure Disease

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Cont. Some cohorts become ambi-directional,

both retrospective and prospective forms.

Such type of design is used when exposedpeople have both short-term and long-termeffects.

Eg Radiation, risks a birth defect within short

time and cancer later.

If large sample size and follow up iscomplete, data from prospective study isreliable and informative when compared to

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eps n a prospec ve co orstudy

1.Define the population at risk (=cohort)

2. Determine exposure status to a factor of interest

of all subjects in the cohort

3. Make sure that study subjects are free of the

disease of interest at time of enrolment

4. Follow exposed and non-exposed forward in

time to ascertain whether they develop theoutcome of interest

5. Compare the outcomes in the exposed and theunexposed group with each other 81

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Retrospective cohort studies1. Well defined population healthy for disease of

interest

2. Selection of study subjects is on the bases of

their exposure status from a record (exposure

status in past)

3. Ascertain outcome (ill or not ill) at present

4. Compare cumulative incidence among exposed

to non-exposed (relative risk)82


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Other classification

Open and closed cohort design

1. Closed (fixed) cohort Members of the cohort will be followed without

adding others.

E.g 1000 children followed for 2 or 3 years

2. Open (Dynamic) cohort study Other than initially chosen study group, others

could be added or be lost.

People at risk are measured using person-time

(incidence density) Data may be analyzed using Time tables and

Kaplan Meir/ Cox regression.

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Open (Dynamic) cohort study

Birth In-migrants

Death Out-migrants

E. g. Butajira Rural Health Program

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.studies

1. Selection of exposed population It depends on a variety ofscientific and other

feasibility considerations, including:

a) The frequency of exposure

(finding sufficient exposed individuals).

Common exposures (Cigarette, coffee drinking etc)

Sufficient exposed people could be found.

Rare exposures (eg occupational/environmentalfactors)

Choose specific groups

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Cont

b. Completeness of follow up

Recruiting stable professionals (Stable

occupation).

Eg. Doctors, Nurses, Veterans, Union members

etc.

In Ethiopia, who are people with stable

occupation?

c. Nature ofresearch questions beingevaluated.

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Cont

2. Selection of comparison (non-exposed)group

Difficult as in the selection of controls in case-

control studies.

Groups should be as similar as possible withrespect to all other factors except exposure

status.

General cohort, and an internal comparison.

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Cont Information obtained from non-exposed should

be adequately comparable with exposed.

In use of rare occupational exposure, we canuse external comparison groups (generalpopulation.).

Comparison from within the institution but not

having exposureeg office workers Vs working in plants.

Comparing with other factories but having nocontact with the exposure.

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Comparison with general rates (such asmortalities, cancer incidents).

Comparison with general population, (haslimitation Healthy worker effect) (Workers arerelatively healthier than the general population).

Compare exposure with other cohort, havingsimilar demography but not exposed.

Use of multiple comparison groups may beother option.

Cont

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5. Source of data

In general we need accurate and complete information on

both exposure and outcome of interest.

1. Exposure ascertainment

a. Pre-existing data:- records from institutions

Advantages;

It is inexpensive.

Provides hard and unbiased information.

Disadvantage;

May not contain detail, adequate and sufficient

information (incomplete).

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b. Interviewing individuals

Able to find information that cant be

recorded

(eg life style of individuals).

Disadvantage;

Bias can be introduced, (recall bias).

Desirability bias

Cont

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Cont

2. Outcome data ascertaining:

For diseases that are fatal, death certificatemay be the source.

For cause specific deaths, it may be obtainedfrom autopsies, physician, verbal autopsiesand hospital records.

For non-fatal end points, physician, hospital

records and even examination by physicians orscreening can be obtained.

NB: Any outcome measurement should be doneEQUALLY both to the exposed as well as non-exposedgroups.

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Follow up is major challenge both cost-wiselyand timely.

Length of follow up depends on the latencyperiod of the outcome. (days, months oryears)

Unless follow up is nearly complete, it isdifficult to interpret. (it may be source of bias).

Therefore, it is crucial to think and achievecomplete follow up.(People are mobile, changing job, changing address

etc).

6. Approaches to follow up

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Issues in Analysis

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Cont.

Calculating incidence of outcomes among

exposed vs non-exposed groups (RR).

Incidences could also be compared

among various levels of exposure and

combinations.

Denominators could be number of

individuals or person-time units. (RR, AR)

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Incidence risk (Cumulative incidence)

Denominator = # of individuals at risk at baseline

Incidence (density) rate

Denominator = person time

Calculate rate ratio (relative risk)

Attributable risk (risk difference)

Cont

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Interpretation

As in other epidemiological studies,interpretation requires the role of Chance,bias, and confounding as explanations offindings.

Role of bias

Selection bias is less concern in cohort

(prospective) studies, but if knowledge ofdisease affects selection of exposed and nonexposed (retrospective), selection bias mayresult.

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7. Role of bias

Any observational study is unlikely to categorizeall individuals correctly.

Misclassification of exposure status or outcomestatus occurs in most cohort studies (two forms).

A. Random (non-differential) misclassification :-inaccuracies occur in similar proportions in eachof the study group.(eg. smocking by # of cigarette Vs quality, pattern of

smocking) Increases similarity between exposed and non-

exposed

This can dilute and could under- or overestimate the results.

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B. Non-random (differential)misclassification:- when misclassificationproduces one sided difference, and resultedin difference of accuracy or quality of

information.

Smoking and bronchitis, (eg. Smokers get more

medical attention, more diagnosed than non-

smokers)

Giving care in ascertaining both exposure

Cont

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8. Effect of loss to follow up

Loss to follow up is the major source of bias incohort studies.

Members of cohort may be lost to follow up, if thisproportion is large, > 20- 25 %, it becomes difficultto validate.

Reduce loss to follow up to an absoluteminimum.

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Losses to follow up: (Solution)

1. Try to find outcome measures from all

possible sources.

2. Compare presence of difference indemographic characteristics between

loss to follow up and follow up

ascertained people,

If no difference then it is not much a

Cont

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Cont3. Indirect estimation of their effect

a. inclusion of assumption of all lose to followup as if they developed the outcome,

b. inclusion of assumption of all loss to followup as if they didnt develop the outcome

This gives the range of association itcould lie.

If the estimate is within the range then it isgood estimation, else it shows that there is

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9. Effects of non-participation

Non-participants (non-response) are participants

who are eligible to participate but who are not

included in the study (non-volunteers).

These who tend to participate differ to those who

did not-participate on motivation, attitude to

health and knowledge of risk status.

The problem encountered by non-response isnot on the internal validity but by difficulty to

generalize (external validity).

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Effect of non-response can be assessed

by comparing basic social and

demographic characteristics of

respondents and non-respondents.

If there is no difference in major

demographic characteristics, then resultsare able to be generalized.

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Summary

Strength

1. Is of particular value when the exposure is rare.

2. Can examine multiple effects of a single exposure.

3. Can elucidate temporal relationship between

exposure and disease.

4. Allows direct measurement of incidence of diseasein the exposed and non exposed.

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Summary

Limitations

1. Is inefficient for the evaluation of rare diseases,

2. If prospective, can be extremelyexpensive and

time consuming.

3. If retrospective, it requires the availability of

adequate records.

4. Validity of results can be seriously affected by

losses to follow up.

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Summary

Cohort studies allow measurement of risk

Case-control studies are rapid, but not

able to measure risk; (only estimate RR)

In the ideal world: Prefer cohort to case-

control study

In the real world: Case-control studiesusually do the job

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Learning objectives

1. Describe the characteristics of a cohort study.

2. Describe the steps and application of cohortstudy design

3. List the conditions under which a cohort study isan appropriate choice to address a researchquestion.

4. List the types of bias most likely to affect a cohortstudy.

epidemiological studies, descriptive 2011 colour less 2 slide per page non-coloured

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