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Table 2 Classification of Level of Evidence (Translated from the Dutch classification of CBO)

From: The effect of aerobic exercise on the number of migraine days, duration and pain intensity in migraine: a systematic literature review and meta-analysis

For articles regarding intervention (prevention or therapy).
A1. Meta-analysis containing at least some trials of level A2 and of which the results of individual trials are consistent.
A2. Randomized comparative clinical trials of good quality (randomized, double-blind controlled trials) of sufficient size and consistency.
B. Randomized controlled trials of moderate (weak) quality or insufficient size or other comparative trials (nonrandomized, cohort studies, patient-control studies)
C. Noncomparative trials
D. Expert opinions