Types of Studies. Aim of epidemiological studies To determine distribution of disease To examine...
-
Upload
debra-smith -
Category
Documents
-
view
217 -
download
0
description
Transcript of Types of Studies. Aim of epidemiological studies To determine distribution of disease To examine...
Types of StudiesTypes of Studies
Aim of epidemiological studiesAim of epidemiological studies
To determine distribution of disease
To examine determinants of a disease
To judge whether a given exposure causes or prevents disease
Epidemiologic Design StrategiesEpidemiologic Design StrategiesDescriptive studies
– Populations Correlated studies
– IndividualsE.g. case-series, case reports, cross-sectional surveys
Analytical studies– Observational studies
Case-control studiesCohort studies
– Intervention studies Clinical trials
Descriptive (person, place, and time)–Hypothesis generating
Analytic (causal)– Hypothesis testing
Descriptive StudiesDescriptive Studies
Correlational studies (may be called ecological studies)– Describes the disease in the interpopulation in
relation to the factor of interest (compare disease frequency between different groups at the same time or the same population at different time).
– Strength Quick and inexpensive, can be used as first step
to measure a disease-exposure relationship– Limitation
Doesn’t link specific persons’ exposure with specific outcome
Can’t control for potential confounding factors
Descriptive StudiesDescriptive StudiesCase Reports and Case Series
– Describes single patient or group of patients experience with similar diagnosis reflecting usually unusual feature of a disease.
– Most common form of study published in medical journals– Strengths
May lead to formulation of new hypotheses Important link between clinical medicine and epidemiology
– LimitationCannot be used to test hypotheses
Descriptive StudiesDescriptive StudiesCross-Sectional (or prevalence) Survey
– Exposure and disease status are simultaneously assessed in a population
– StrengthProvides information about the frequency and characteristics of a
disease– Useful for public health– Can provide information concerning the prevalence of disease or
other health outcome in special groups (e.g. occupations)– Limitation
Can’t determine whether exposure preceded or occurred as a result of the disease
Analytic StudiesAnalytic StudiesExplicit comparison of exposure and diseaseGroups are assembled to determine whether risk
is different for exposed and unexposedAppropriate comparison groupHypothesis testingTwo types
– Observational - natural course of events– Intervention - investigator allocates exposure and
follows subjects
Observational StudiesObservational StudiesCase-Control
– Persons with disease– Comparison group
Cohort– Subjects classified on basis of exposure of a
factor– Follow-up to determine presence of disease
Prospective vs. retrospective
Case-Control studiesCase-Control studiesSubjects are selected on the basis of
whether they do (cases) or don’t (controls) have a particular disease under study, the groups are then compared with respect to proportion having a history of unexposure or characteristic interest.
CASE-CONTROL STUDIES CASE-CONTROL STUDIES (Cont.)(Cont.)
Advantages– Uniquely suited to diseases with long incubation periods– More efficient in terms of time and money– Good for study of rare disease– Can look at multiple exposures for a single disease
Disadvantages– Inefficient for evaluation of rare exposures – Cannot directly compute incidence rates of disease– Particularly prone to bias (selection and recall in particular)
Cohort StudiesCohort StudiesA cohort study is a study where a group of
individuals are followed. The study population is defined on basis on the presence or absence of exposure to a suspected risk factor for a disease.
•Cohort study is undertaken to support the Cohort study is undertaken to support the existence of association between suspected existence of association between suspected cause and diseasecause and disease A major limitation of cross-sectional surveys and
case-control studies is difficulty in determining if exposure or risk factor preceded the disease or outcome.
Cohort Study:
Key Point:
– Presence or absence of risk factor is determined before outcome occurs.
Cohort studiesCohort studies– longitudinal – Prospective studies– Forward looking study I– Incidence study
starts with people free of disease assesses exposure at “baseline”assesses disease status at “follow-up”
Cohort studiesCohort studies Strengths We can find out
incidence rate and risk More than one disease
related to single exposure
can establish cause - effect
good when exposure is rare
minimizes selection and information bias
Weaknesses losses to follow-up often requires large
sample ineffective for rare
diseases long time to complete expensive Ethical issues
INTERVENTIONAL STUDYINTERVENTIONAL STUDY
Interventional studies are when researchers attempt to show that A causes B by actually manipulating those factors they think have something to do with causing some outcome.
Intervention Studies - explore the association between interventions and outcomes. (Experimental studies or clinical trials)
Examples of Experimental Examples of Experimental Epidemiologic StudiesEpidemiologic Studies
Prophylactic vaccines tested on children populations to prove the efficacy of the vaccines in preventing the diseases (i.e., polio)
Prophylaxis with drugs in preventing disease (i.e., penicillin to prevent rheumatic fever)
Impact on health-related behavior and coronary heart disease in response to community-wide heart disease prevention intervention
Experimental Study DesignExperimental Study Design
Time
Sample of Cases
Treated (T)
Not Treated (NT)(Control)
Treated - Improved
Treated – Not Improved
Not Treated - Improved
Not Treated – Not Improved
Interventional StudyInterventional StudyOn lab animalsClinical Trials: 1.single-blinded 2. double-blinded
AdvantagesAdvantagesHelpful in assessing the value of new
therapies to combat acute diseases in developing countries
Prospective designEliminates bias by comparing two
otherwise identical groupsAllows for meta-analysis
DisadvantagesDisadvantagesExpensive and time consumingNot always properly conducted – too
few subjects, too short a time period