Faculty of Medicine, Dentistry and Health Sciences
UWA Medical and Dental Library
Study types summary
Study Type Clinical Question Evidence Strength Sources of Bias Timing
RCT Therapy/Intervention
Intervention
Aetiology
(unethical?)
Strongest Known and
unknown
confounders equally
distributed in groups
• No allocation concealment
• loss to follow analysis by intention to
treat
• non-blinded outcomes assessment
Begins with study
factor – randomized
exposure
Cohort study Therapy/Intervention
Aetiology
Prognosis
Observational • Loss to follow up
• non-blind assessment of outcome
• confounding
Begins with study
factor exposure, non
randomized
Case-control
study
Therapy/Intervention
Aetiology
Observational Good for rare
diseases
• Recall bias
• Confounding
• low response rates
• non-blind assessment of study factor’
controls not chosen from same
population as cases
Begins with the
outcome factor
Cross-sectional
analytic
(Diagnostic)
Diagnosis Best study to assess
diagnostic test
accuracy
• Non-independent blind comparison
with reference standard
• verification bias
• inappropriate spectrum of disease
Compares a diagnostic
test to a gold standard
Cross-sectional
analytic
Therapy/Intervention
Aetiology
Association/hypothesis
generating
Study several
study/outcome
factors at one time
to assess association
• Selection of participants non-random
(volunteers)
• Confounding
• low response rates
• small sample sizes
Study and outcome
factors measured at
one time point