This document compares and contrasts different types of epidemiological study designs: randomized controlled trials (RCT), cohort studies, case-control studies, case series, and cross-sectional studies. It provides information on key aspects of each type of study such as whether they include a control group, difficulty level, costs, biases, whether they are prospective or retrospective, time required, need for follow up, and strengths and limitations for different research objectives. RCTs are best for examining the effects of interventions while cohort studies can determine risk factors by observing exposure and outcome over time in test and control groups. Case-control studies are useful for rare diseases but prone to biases. Case series report unique findings but lack controls. Cross-sectional studies measure

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Case Report RCT Cohort Case Control Case Series
Cross-sectional
Study
Control
Group
No Yes No Yes No No
Difficulty Low Difficult Difficult Moderate Moderate Low
Cost Cheap Expensive Expensive Cheap Moderate Cheap
Bias/ non-
bias
Bias
Non-bias
(minimizes
allocation bias,
selection bias)
Bias
(prone to
attrition bias,
bias of
change in
methods of
time,
Diagnostic
bias)
Bias
(prone to
selection bias,
Non-response
bias, recall bias,
observer bias)
Bias
(selection bias,
Non-response
bias,
Observation
bias)
Bias/ non-bias
Pro-/Retro-
spective
Retrospective Prospective Prospective Retrospective
Pro-/Retro-
spective
Prospective
Good For
Generally report a
new or unique
finding. Eg:
• Previously
undescribed
disease
• Unexpected
link between
Used to examine
the effect of
interventions
(including
treatment,
prevention
strategies,
screening
Wide range
of exposure-
disease
studies.
Used to
determine risk
factors, on
exposure of the
patients and
control group
equally, for the
same period of
The study of
new or unusual
disease or
treatment.
Often used to study
conditions that are
relatively frequent
with long duration of
expression.

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diseases
• Unexpected
new
therapeutic
effect
• Adverse events.
programs and
diagnostic tests)
on particular
outcomes such
as death or
reoccurrence of
disease.
time.
Time Short Long Long Short Short Short
Need for
Follow Up
No Yes Yes Yes Yes No
Summary &
Comments
A quick
communication.
A report based on
a single case of a
condition. Often a
rare condition.
Sometimes
collected together
into a short series.
Aims to
evaluate
efficacy of a
new
medication or
treatment
modality.
The Control
and Test
groups should
be similar in
terms of other
significant
factors (sex,
age, other
known risk
factors etc).
Aim is to
observe
changes
related to
a
particular
group of
individuals
.
A case-
control study,
although
retrospective
, is superior
to a case
series
because of
the presence
of a control
group.
Certain types
of study bias
are unique to
case-control
Studies (eg:
hospital
bias).
A Case series
may be a study
that samples
patients with
both, a specific
outcome and a
specific
exposure, or
one that
samples patients
with a specific
outcome and
includes
patients
regardless of
whether they
have specific
exposures.
A Cross-sectional
study is an
“observational”
design that
surveys exposures
and disease
status at a single
point in time.
It measures
prevalence, not
incidence of
disease.
Not suitable for
studying rare or
highly fatal
diseases or a
disease with short
duration of
expression.

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Resources:
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 Abdulla, Nizam. “Evidence-Based Dentistry”. Australia, Adelaide: The University of Adelaide, School of Dentistry.
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Dec2004, Volume 183, Number 6.
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Hutchinson Cancer Research Center, Seattle, WA, USA. Epidemiol Rev1998;20(1):43-56.
 Hess DR. “Retrospective studies and chart reviews.”Respiratory Care, Ellison 401, Massachusetts General Hospital, 55 Fruit Street,
Boston, MA 02114, USA.2004 Oct;49(10):1171-4.
 Dekkers OM, Egger M, Altman DG, Vandenbroucke JP. “Distinguish case series from cohort studies.” Leiden University Medical
Center, the Netherlands. Ann Intern Med. 2012 Jan 3;156(1 Pt 1):37-40.