Fig. 3

Forest plot of the 27 studies included in the quantitative synthesis for preventive migraine treatment (Panel a, at least two studies. Panel b, single studies). The common outcome was the reduction of headache days in the active treatment and control groups (the higher the reduction, the more effective the treatment). Studies using the same techniques are pooled together. All references for studies in this figure are listed in the online supplementary references. SMD: standardized mean difference, tDCS: transcranial direct current stimulation. Three studies were not included in the quantitative synthesis: Allais et al., [23] (3 TENS devices); Bono et al., [24] (occipital TENS); Jia et al., [25] (PENS); Kosari et al., [26] (tDCS over the visual cortex)