The normal distribution is not normal in psychological data: Moving beyond parametric dogma

Heury Ferr

PLOS Mental Health, 2025, vol. 2, issue 8, 1-4

Abstract: Psychological research has traditionally relied on parametric statistical methods, largely due to the historical convenience of the normal distribution. However, empirical evidence shows that psychological and mental health data often violate normality assumptions, exhibiting skewness, kurtosis, ordinal scaling, and outliers. Common constructs such as stress, anxiety, and substance use frequently display zero-inflated or asymmetric distributions, making parametric methods inappropriate and potentially misleading. Violations of normality increase the risk of Type I and II errors, bias effect estimates, and undermine inferential validity. While non-parametric tests offer more robust alternatives, modern resampling techniques such as bootstrapping and Monte Carlo simulations provide greater flexibility and accuracy without relying on strict distributional assumptions. This paper illustrates common deviations from normality in psychological data and advocates for a paradigm shift toward assumption-light analytical strategies. Emphasizing data visualization, transparent reporting, and statistical education, we argue for broader adoption of flexible methods to ensure more valid, interpretable, and reproducible findings in psychological science.

Date: 2025
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Persistent link: https://EconPapers.repec.org/RePEc:plo:pmen00:0000403

DOI: 10.1371/journal.pmen.0000403

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