epidemiological designs 13

8/12/2019 Epidemiological Designs 13

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Epidemiological Designs


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Learning Objectives

By the end of this chapter, the reader will beable to:

Distinguish between observational and

experimental studies. Describe the key characteristics of experimental,

cohort, casecontrol, cross-sectional, and ecologicstudies regarding subject selection, data

collection, and analysis. Discuss the factors that determine when a

particular design is indicated.


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Introduction

Epidemiology is the study of the distributionand determinants of disease frequency inhuman populations and the application of this

study to control health problems. Epidemiology is the study of the occurrence

and spread of diseases in specifiedpopulations.

Concerned with the health status (or moreusually the ill-health status of populations.


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Introduction

The term study includes both surveillance,

whose purpose is to monitor aspects of

disease occurrence and spread that are

pertinent to effective control, and

Epidemiologic research, whose goal is to

harvest valid and precise information about

the causes, preventions, and treatments fordisease


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Introduction

In epidemiology, measuring the occurrence ofdisease or other health related events in apopulation is only a beginning.

Epidemiologists are also interested inassessing whether an exposure is associatedwith a particular disease (or other outcome ofinterest).

For instance, researchers may be interested inobtaining answers to the following questions:


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Introduction

Does a high-fat diet increase the risk of breast

cancer?

High-fat diet Breast cancer

(exposure) (outcome)

Does hepatitis B virus infection increase the

risk of liver cancer?

Hepatitis B infection Liver cancer

(exposure) (outcome)


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Introduction

The first step in an epidemiological study is to

define the hypothesis to be tested.

This should include a precise definition of the

exposure(s) and outcome(s) under study.

The next step is to decide which study design

will be the most appropriate to test that

specific study hypothesis.


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Study Designs

The overall plan used in gathering data to

answer research questions or test research

hypotheses.

Specific strategy employed in undertaking the

research.

Provides the framework on which procedures

for the research are specified and conducted.


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Study Designs

Epidemiologic research encompasses several

types of study designs, including experimental

studies and observational studies such as

cohort and casecontrol studies.

Each type of epidemiologic study design

simply represents a different way of

harvesting information.


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Study Designs

The decision for a specific type of design

depends on

The type of problem

Knowledge available about the problem

Resources available(cost)

Speed

Availability of data

Other criteria include research hypotheses


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Study Designs

The decision for a specific type of design

depends on

The type of problem

Knowledge available about the problem

Resources available(cost)

Speed

Availability of data

Other criteria include research hypotheses


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The selection of one design over another

depends on the particular research question,

concerns about validity and efficiency, and

practical and ethical considerations.

Validityis the degree to which an instrument

measures what it is interned to measure.


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Internal validityis the degree to which it can

be inferred that the experimental treatment

or independent variable, rather than

uncontrolled extraneous factors is responsible

for the effect on the dependent variable.


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Example, the extent to which it can be

deduced that, exclusive breast feeding is

responsible for proper growth and

development of a baby than other factors.

External validityis the degree to which the

results of a study can be generalized to

settings or samples other than the onesstudied.


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Uses of epidemiological studies

Diagnostic purposes

Community diagnosis or group diagnoses is made

from epidemiological studies.

Provides information about the determinants of

health in the specific community or group.

Community diagnosis is necessary for establishing

the basis for intervention. Information is used for planning of health

services.


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Provision of information on etiology

Through epidemiological studies,

hypotheses on disease causation or

associated factors can be formulated and

tested.

The risk of developing a disease is based on

the incidence rates in the general

population or among a sub-group can be

estimated.


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Determination of the natural history (Course

of disease over time)

Knowledge of the natural history enables the

prediction of the probability of the outcome of

diseases (prognosis).

Also assist in determining at what stage

interventions will be of benefit. E.g Screening for early detection of certain

condition such cancer of the cervix.


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Contribution to evaluation of health care

Epidemiological studies contribute to evaluationof health care in both specific local and general

situations through experimental, quasi-experimental or non experimental designs

Specific

How well is this sexuality education for adolescence

working? General

Does the vaccine prevent disease?


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Epidemiological studies answer the ff

questions

What is the state of health of the community?

What are the factors responsible for this state of

health?

What is being done about the health system and

the community itself?


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What more can be done and what is the expected

outcome?

What measures are needed to continue healthsurveillance of the community and to evaluate the

effects of what is being done?


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Types of Study Designs

There are two basic approaches to assessing

whether an exposure is associated with a

particular outcome:

Experimental and Observational.


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Experimental studies

These are studies in which the researchermanipulates objects (situation) and measuresthe effect of his manipulation.

Usually, but not always, two groups arecompared.

One group in which the intervention takes

place example treatment with a certain drugand another group at remain untouched,example treatment with placebo.


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This is a study in which the experimental

group is always exposed to an intervention.

Intervention studies have all or same

combinations of the following characteristics;

Manipulation

Control

Randomization


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Manipulation:

This is when the researcher does something to

one group of subjects in the study that is

manipulate or introduces an intervention.

Example In the management of post operative

pain: Injection Pethidine may be given to the

experimental group whilst water for injection

is given to the control group.


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Control:

This is a group comparable to the

experimented group in every way except the

variable being tested.

That is possessing similar characteristics with

the experimental group but will receive water

for injection instead of Pethidine.


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Randomization:

This is the choice of subjects into both groups

(experimental and control group).

Selection is done randomly or the random

solution of subject into the experimental and

control groups.

This is a choice of subjects in both groups and

it is done randomly.


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Observational Studies

These are studies in which the researcher just

describes and analyses researchable objects or

situation but does not intervene.

The researcher collects information on attributesof interest but the experimental method is not

used.

There is no active intervention by the investigator.

Investigators study people and exposures in

nature.


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Descriptive Epidemiological Studies

Observational studies which study the amount

(frequency) and distribution (by person, place

and time) of disease events or events within a

population.

Objectives of descriptive studies

To provide information on or evaluate trends in

health and make comparisons between groups.

To provide information for planning and

evaluating of services.


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To generate hypotheses to be tested byanalytical or experimental methods.

Characteristics of a disease studied are with

reference to the Person, place and timefactors

Who (person) is affected?

Where (place) does it occur?

When (time) does it occur.

seasonal patterns, secular trends, or acute changes indisease occurrence (i.e., epidemic)


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Descriptive studies provide information on patterns ofdisease occurrence

The descriptive statistics generated can be correlatedwith clinical observations or laboratory studies to

generate hypotheses

Often provide clues about disease causation that canbe pursued by more sophisticated research designs

Analytic studies are designed specifically to testhypotheses that have usually been generated from

descriptive studies


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Case Report

Clinical case report are descriptiveepidemiological studies.

Usually reports on unusual disease (e.g

unusual mode of presentation. It is termed a case and is expected to generate

information for further investigation.

E.g Acute Encephalopathy associated withMetronidazole therapy in a patient with liverabscess.


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Case series Report

An aggregate of individuals cases is said toconstitute case series.

It generates more reaction than a case report.

Single report of an unusual case might nottrigger as much reaction as a report of severalunusual cases.

E.g A single case of gastroenteritis (mimickingfood poisoning) might not generate so muchconcern.


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But several cases are report within a short

period, it would alarm health workers and

trigger of reactions for initiation of research.

It will direct attention to an outbreak of food

poisoning.

Case Reports and Case Series

Describe the experience of a single patient or a

group of patients with similar diagnosis.


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Typically, an observant clinician reports an

unusual feature of a disease, a patients

exposure history, or unusual medical event

May lead to formulation of new

hypotheses

A series of unusual cases may prompt

further investigations with more rigorous

study designs


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Advantages and Disadvantages

Advantages

Useful in the formulation of research

hypotheses

suggestive of risk factors

Important step in recognizing new diseases

or risk factors.


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Disadvantages

Case report is based on the experience of one

individual

The presence of any risk factor may be

coincidental

Cant use to test for valid statistical association

(No comparison group)


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Cross-sectional study

A survey in which measurements (datacollection) are done as a single observation.

Descriptive cross sectional study sets out to

describe situations. E.g Distribution of an even in a population in

relation to age, sex, and other specifiedcharacteristics.

In a study of the prevalence study of contraceptiveuse among a defined population.


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Cross-sectional study

These studies are designed if the researcherwants to draw conclusions from findings tocover a larger population.

This study is more extensive than casestudies and focuses on describing groups.

It aims at quantifying the distribution ofcertain variables in a defined study populationat one point in time.

These characteristics may include:


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Physical characteristics of people, materials orthe environment as in Prevalence surveys (of bilhazia, leprosy)

Evaluation of coverage (of immunization)

Socioeconomic characteristics like age,education, mental status, number of children andincome

The behavior of the people and the knowledge,

attitude, behoviours and opinions that may helpto explain that behavior. (KAP studies)

Events that occurred in the population.


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Since both exposure and disease are measured at

the same time, it is difficult to ascertain temporal

relationship.

That is which preceded the other

Examples of cross-sectional studies

Determinants of Health care Utilization

Knowledge, Attitudes and Practices of growthMonitoring among mothers in two rural communities.

Knowledge and practice of CPR among Nurses


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Characteristics of Cross-Sectional

Studies

Exposure and disease outcome are

determined simultaneously for each subject

Identify prevalent cases (the cases existed at

the time of the study, but do not know their

duration)

Measure prevalence, not incidence (new

cases)

Also called a prevalence study


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Prevalence is a function of both incidence and

duration of disease

Prevalence = Incidence rate x Duration of

disease

So measures of association based on

prevalent cases reflect both

The exposures effect on incidence

The exposures effect on duration or survival

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