Abstract
The Importance of Sample Size for Reproducibility of tDCS Effects.
Highlights
Cheap, easy to apply, well-tolerable, with the potential of altering cortical excitability, and for testing causalities—these are attributes that have made transcranial direct current stimulation a highly popular research tool in cognitive neuroscience
Depending on which studies are included in systematic reviews and meta- analyses, small sample size in transcranial direct current stimulation (tDCS) research could lead to both under—and overestimation of tDCS efficacy
RT was significantly different between anodal and cathodal tDCS [t(73) = −1.91, p = 0.03 [one-tailed], d = 0.45], with anodal stimulation resulting in faster RTs than cathodal tDCS
Summary
Easy to apply, well-tolerable, with the potential of altering cortical excitability, and for testing causalities—these are attributes that have made transcranial direct current stimulation (tDCS) a highly popular research tool in cognitive neuroscience. One potential reason for the reported inconsistencies might be that sample size is usually very small in most tDCS studies (including those from our research group) Whilst this issue is not specific to tDCS studies ( Button et al, 2013 estimate the median statistical power in neuroscience in general being only 21%), it could lead to weaker effects often not being detected, and meta- analyses suggesting small or no efficacy of tDCS. Depending on which studies are included in systematic reviews and meta- analyses (i.e., findings published in peer-reviewed journals; unpublished nil-effects; nil-effects reported as an additional finding in papers with the actual focus on another, significant, effect, etc.), small sample size in tDCS research could lead to both under—and overestimation of tDCS efficacy. Taking publication bias into account it becomes evident that efficacy of tDCS is rather weak (Mancuso et al, 2016)
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