Based on a multicenter double-blind crossover prophylatic migraine trial (n = 96), the relative effectiveness of the crossover design versus group comparison was evaluated. If a ‘time effect’ is taken into account in the statistical model, the crossover design was estimated to be 10–11 times more effective than the group comparison. With more traditional statistical methods using paired and unpaired t tests, the crossover design was estimated to be 4 times more powerful than the group comparison. Based on the t test, sample size tables for numbers of patients needed for detecting effect differences from 10 to 50 with powers from 0.6 to 0.9 in a crossover and a group comparison are presented. These calculations are based on only one trial, and it is recommended that similar estimations of sample variability should be done in other previous t
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