epidemiological designs 13
TRANSCRIPT
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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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Uses of epidemiological studies
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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Uses of epidemiological studies
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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Uses of epidemiological studies
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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Uses of epidemiological studies
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