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