Types of Studies
Aim 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 Strategies

Descriptive studies
– Populations

Correlated studies
– Individuals


E.g. case-series, case reports, cross-sectional
surveys
Analytical studies
– Observational studies
Case-control studies
 Cohort studies

– Intervention studies

Clinical trials
 Descriptive
(person, place, and
time)
– Hypothesis generating
 Analytic
(causal)
– Hypothesis testing
Descriptive 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 Studies

Case 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

– Limitation

Cannot be used to test hypotheses
Descriptive Studies

Cross-Sectional (or prevalence) Survey
– Exposure and disease status are simultaneously
assessed in a population
– Strength

Provides 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 Studies
Explicit comparison of exposure and
disease
 Groups are assembled to determine whether
risk is different for exposed and unexposed
 Appropriate comparison group
 Hypothesis testing
 Two types

– Observational - natural course of events
– Intervention - investigator allocates exposure
and follows subjects
Observational Studies

Case-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 studies

Subjects 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
(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 Studies

A 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 •
existence of association between
suspected cause 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 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 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 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
Epidemiologic 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 Design
Treated - Improved
Time
Treated (T)
Sample of
Cases
Treated – Not Improved
Not Treated - Improved
Not Treated (NT)
(Control)
Not Treated – Not Improved
Interventional Study
On lab animals
 Clinical Trials: 1.single-blinded
2. double-blinded

Advantages
Helpful in assessing the value of new
therapies to combat acute diseases in
developing countries
 Prospective design
 Eliminates bias by comparing two
otherwise identical groups
 Allows for meta-analysis

Disadvantages
Expensive and time consuming
 Not always properly conducted – too
few subjects, too short a time period
