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Centre for Evidence-
Based Medicine

Are the results of this study valid?

Information about prognosis can come from a variety of study types. Cohort studies (investigators follow 1 or more groups of individuals over time and monitor for the occurrence of the outcome of interest) are the best source of evidence about prognosis. Randomised control trials can also provide information about prognosis although trial participants may not be representative of the population with the disorder. Case-control studies (investigators retrospectively determine prognostic factors by defining the exposure of cases who have already experienced the outcome of interest and of controls who haven't) are useful when the outcome of interest is rare or when the required follow-up is long. The strength of inference that can be drawn from a case-control study is limited because they are more susceptible to bias.

Returning to our clinical scenario from the question formulation tutorial:

You see a 70 year old man in your outpatient clinic 3 months after he was discharged from your service with an ischemic stroke. He is in sinus rhythm, has mild residual left-sided weakness but is otherwise well. His only medication is ASA and he has no allergies. He recently saw an article on the BMJ website describing the risk of seizure after a stroke and is concerned that this will happen to him.

In the tutorial on clinical questions, we formulated the following question: In a 70 year old man does a history of stroke increase his risk for seizure?

Our search of the literature to answer this question retrieved an article from the BMJ (1997;315:1582-7).

How do we critically appraise this prognosis paper? We'll start by considering validity first and the following list outlines the questions that we need to consider when deciding if a prognosis paper is valid.

  1. Was a defined, representative sample of patients assembled at a common (usually early) point in the course of their disease?
  2. Was patient follow-up sufficiently long and complete?
  3. Were objective outcome criteria applied in a "blind" fashion?
  4. If subgroups with different prognoses are identified:

Do you want to review validity or importance?