Variability by region and method in human brain sodium concentrations estimated by ²³Na magnetic resonance imaging: a meta-analysis.

In GM the test of asymmetry was at the threshold of significance (t = 2.1, p = 0.05), though after removing some outliers (60 mM 45 ; 69 mM 5 ) the figures become more symmetrical and the tests become non-significant (t = 1.73, p = 0.1) and the remaining regions do not show significant skewness.

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Asymmetry can be evidence for small study effects and publication bias, based on the assumption that small studies are at greatest risk of non-significant results and biasing the published literature toward the high effect sizes that are most likely to be significant with small N 43 .

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