An Exploration of Variability Due to Low Power in Structural MRI Studies of Bilingualism



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The adequacy of replicability among psychological findings has previously been questioned, especially for neuroscientific fields of research. Researchers increasingly point towards the negative effects of low power on replicability of findings. Though decreased sensitivity in smaller samples is a well-known consequence of inadequate power, many overlook the increased likelihood of inflated observed effects and weakened positive predictive values. The aim of this study is to reveal the expected degrees of uncertainty among neuroimaging findings by conducting tests in different sample sizes from a larger-than-average sample, in an area of research with wide-ranging findings that have been proposed by some to be due in part to inadequate sample sizes: bilingual-monolingual structural brain differences. Bilinguals (n = 216) were compared with monolinguals (n = 146) using grey matter density in whole-brain analyses and grey matter volume measures across region-of-interest tests. Variability among findings were compared with the true full-sample findings, and taken in the context of expected differences within the larger bilingualism neuroimaging literature. Results demonstrate excessive variability across the lowest sample sizes (e.g. samples totaling 20 – 80 participants), and this is explored through the trends of subsample outcomes and effect sizes across sample sizes. The extent to which infrequently utilized methods such as multivariate analyses of covariance (MANCOVAs) and Bayes Factors can improve the accuracy of results at lower sample sizes were also explored. It is our hope that this study helps to demonstrate the influences of power on expected variability among sample findings, especially for bilingual researchers and any researchers interested in exploring group differences using neuroimaging.



Replicability, Sample size, Power, Neuroimaging, MRI, Bilingualism