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Centre for Evidence-
Based Medicine

Were patients and clinicians kept blind to treatment? And, were groups treated equally, apart from the experimental therapy?

Blinding of clinicians and patients helps to prevent additional treatment. The provision of treatment (received in addition to the experimental treatment) to just one of the groups is called cointervention. If either the patients or the clinicians weren't blinded it could lead to the reporting of symptoms or the interpretation of these symptoms to be affected by suspicion about the effectiveness of the treatment under investigation.

In the NASCET study, all patients received antiplatelet therapy (this was usually ASA and the dose was left to the discretion of the neurologist at each study centre), and when indicated they received antihypertensive and or antilipidemic medications.

Blinding is not always possible (such as in surgery trials) and in these situations we should check to see if outcome events were assessed by blinded investigators. For example in NASCET, outcome events were assessed by 4 groups: the participating neurologist and surgeon; the neurologist at the study centre; by 'blinded' members of the steering committee; and by 'blinded' external adjudicators.

  1. Was the assignment of patients to treatment randomized? And, was the randomization list concealed?
  2. Was follow-up of patients sufficiently long and complete?
  3. Were all patients analyzed in the groups to which they were randomized?

    And some less important points:
  4. Were patients and clinicians kept blind to treatment?
  5. Were groups treated equally, apart from the experimental therapy?
  6. Were the groups similar at the start of the trial?