Study Designs

Afshin Ostovar

Bushehr University of Medical Sciences

Bushehr, 2012

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Definition of Epidemiology

The word epidemiology comes from the Greek words:

epi, meaning “on or upon” demos, meaning “people” logos, meaning “the study of”

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Definition of Epidemiology

Epidemiology is:

“the study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to the control of health problems.”

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Definition of Epidemiology

Epidemiology is:

“the study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to the control of health problems.”

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Study:

Epidemiology is a scientific discipline, sometimes called “the basic science of public health.” It has, at its foundation, sound methods of scientific inquiry.

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Distribution:

Epidemiology is concerned with the frequency and pattern of health events in a population.

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Frequency includes not only the number of such events in a population, but also the rate or risk of disease in the population.

The rate (number of events divided by size of the population) is critical to epidemiologists because it allows valid comparisons across different populations.

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Pattern refers to the occurrence of health-related events by time, place, and personal characteristics.

Time characteristics include annual occurrence, seasonal occurrence, and daily or even hourly occurrence during an epidemic.

Place characteristics include geographic variation, urban-rural differences, and location of worksites or schools.

Personal characteristics include demographic factors such as age, race, sex, marital status, and socioeconomic status, as well as behaviors and environmental exposures.

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This characterization of the distribution of health-related states or events is one broad aspect of epidemiology called descriptive epidemiology. Descriptive epidemiology provides the What, Who, When, and Where of health-related events.

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Determinants:

Epidemiology is also used to search for causes and other factors that influence the occurrence of health-related events.

Analytic epidemiology attempts to provide the Why and How of such events by comparing groups with different rates of disease occurrence and with differences in demographic characteristics, genetic or immunologic make-up, behaviors, environmental exposures, and other so-called potential risk factors.

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Health-related states or events:

Epidemics of communicable diseases.

Endemic communicable diseases and non-communicable infectious diseases.

Chronic diseases, injuries, birth defects, maternal-child health, occupational health, and environmental health.

Behaviors related to health and well-being (amount of exercise, seat-belt use, etc.)

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Application:

Epidemiology is more than “the study of.”

As a discipline within public health, epidemiology provides data for directing public health action.

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Definition of Epidemiology

Epidemiology is “the study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to the control” of health problems.

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Uses of epidemiology

Population or community health assessment

Individual decisions

Completing the clinical picture

Search for causes

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Study Designs

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Descriptive Vs Analytical A descriptive study is limited to a description of the

occurrence of a disease in a population and is often the first step in an epidemiological investigation. Pure descriptive studies make no attempt to analyse the links between exposure and effect.

An analytical study goes further by analysing relationships between health status and other variables.

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A classic example of Descriptive study

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Case Report Case reports represent the most basic type of

study design, in which researchers describe the experience of a single person.

Case reports describe individuals who develop a particular new disease or condition and present a detailed account of the clinical experience of an individual study subject.

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Case Series The characteristics of several patients with a

specific disease are described but are not compared with those of a reference population.

Often stimulates the initiation of a more detailed epidemiological study.

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Ecological studies In an ecological study, the units of analysis are

groups of people rather than individuals.

Ecological studies can also be done by comparing populations in different places at the same time or, in a time series, by comparing the same population in one place at different times.

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Ecological fallacy An ecological fallacy or bias results if

inappropriate conclusions are drawn on the basis of ecological data.

The bias occurs because the association observed between variables at the group level does not necessarily represent the association that exists at the individual level

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Cross-sectional studies Measure the prevalence of disease and thus are

often called prevalence studies.

The measurements of exposure and effect are made at the same time.

The key question to be asked is whether the exposure precedes or follows the effect.

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Cross-sectional studies Relatively easy and inexpensive to conduct

Useful for investigating exposures that are fixed characteristics of individuals, such as ethnicity or blood group.

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Case-control studies Provide a relatively simple way to investigate

causes of diseases, especially rare diseases.

They include people with a disease (or other outcome variable) of interest and a suitable control (comparison or reference) group of people unaffected by the disease or outcome variable.

The study compares the occurrence of the possible cause in cases and in controls.

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Cohort studies Also called follow-up or incidence studies, begin

with a group of people who are free of disease, and who are classified into subgroups according to exposure to a potential cause of disease or outcome.

Variables of interest are specified and measured and the whole cohort is followed up to see how the subsequent development of new cases of the disease (or other outcome) differs between the groups with and without exposure.

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Randomized controlled trials

A randomized controlled trial is an epidemiological experiment designed to study the effects of a particular intervention.

Subjects in the study population are randomly allocated to intervention and control groups, and the results are assessed by comparing outcomes.

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Field trials Field trials, in contrast to clinical trials, involve people

who are healthy but presumed to be at risk.

Data collection takes place “in the field,” usually among non-institutionalized people in the general population.

Since the subjects are disease free and the purpose is to prevent diseases that may occur with relatively low frequency, field trials are often logistically complicated and expensive endeavours.04/19/23 35

Community trials In this form of experiment, the treatment groups

are communities rather than individuals.

This is particularly appropriate for diseases that are influenced by social conditions, and for which prevention efforts target group behaviour.

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Community trials (Limitations)

Only a small number of communities can be included and random allocation of communities is usually not practicable.

Other methods are required to ensure that any differences found at the end of the study can be attributed to the intervention rather than to inherent differences between communities.

It is difficult to isolate the communities where intervention is taking place from general social changes that may be occurring.04/19/23 37

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