Study design Flashcards
Participants randomly allocated to intervention or control group (e.g. standard treatment or placebo)
Practical or ethical problems may limit use
Randomised controlled trial
Observational and prospective. Two (or more) are selected according to their exposure to a particular agent (e.g. medicine, toxin) and followed up to see how many develop a disease or other outcome.
The usual outcome measure is the relative risk.
Examples include Framingham Heart Study
Cohort study
Observational and retrospective. Patients with a particular condition (cases) are identified and matched with controls. Data is then collected on past exposure to a possible causal agent for the condition.
The usual outcome measure is the odds ratio.
Inexpensive, produce quick results
Useful for studying rare conditions
Prone to confounding
Case-control study
Provide a ‘snapshot’, sometimes called prevalence studies
Provide weak evidence of cause and effect
Cross-sectional survey
Levels of evidence
Ia - evidence from meta-analysis of randomised controlled trials
Ib - evidence from at least one randomised controlled trial
IIa - evidence from at least one well designed controlled trial which is not randomised
IIb - evidence from at least one well designed experimental trial
III - evidence from case, correlation and comparative studies
IV - evidence from a panel of experts
Grading of recommendation
Grade A - based on evidence from at least one randomised controlled trial (i.e. Ia or Ib)
Grade B - based on evidence from non-randomised controlled trials (i.e. IIa, IIb or III)
Grade C - based on evidence from a panel of experts (i.e. IV)
When testing a new drug compared to an existing treatment, a trial must show one of 3 outcomes
Superiority (a large sample size needed to show a significant benefit over an existing treatment)
Equivalence (drugs may be assumed to have a similar effect)
Non-inferiority (from small trials. Once a drug has been shown to be non-inferior, large studies may be performed to show superiority)
