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Outlier Removal and the Relation with Reporting Errors and Quality of Psychological Research

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  • Marjan Bakker
  • Jelte M Wicherts

Abstract

Background: The removal of outliers to acquire a significant result is a questionable research practice that appears to be commonly used in psychology. In this study, we investigated whether the removal of outliers in psychology papers is related to weaker evidence (against the null hypothesis of no effect), a higher prevalence of reporting errors, and smaller sample sizes in these papers compared to papers in the same journals that did not report the exclusion of outliers from the analyses. Methods and Findings: We retrieved a total of 2667 statistical results of null hypothesis significance tests from 153 articles in main psychology journals, and compared results from articles in which outliers were removed (N = 92) with results from articles that reported no exclusion of outliers (N = 61). We preregistered our hypotheses and methods and analyzed the data at the level of articles. Results show no significant difference between the two types of articles in median p value, sample sizes, or prevalence of all reporting errors, large reporting errors, and reporting errors that concerned the statistical significance. However, we did find a discrepancy between the reported degrees of freedom of t tests and the reported sample size in 41% of articles that did not report removal of any data values. This suggests common failure to report data exclusions (or missingness) in psychological articles. Conclusions: We failed to find that the removal of outliers from the analysis in psychological articles was related to weaker evidence (against the null hypothesis of no effect), sample size, or the prevalence of errors. However, our control sample might be contaminated due to nondisclosure of excluded values in articles that did not report exclusion of outliers. Results therefore highlight the importance of more transparent reporting of statistical analyses.

Suggested Citation

  • Marjan Bakker & Jelte M Wicherts, 2014. "Outlier Removal and the Relation with Reporting Errors and Quality of Psychological Research," PLOS ONE, Public Library of Science, vol. 9(7), pages 1-9, July.
  • Handle: RePEc:plo:pone00:0103360
    DOI: 10.1371/journal.pone.0103360
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    1. Bruns, Stephan & Herwartz, Helmut & Ioannidis, John P.A. & Islam, Chris-Gabriel & Raters, Fabian H. C., 2023. "Statistical reporting errors in economics," MetaArXiv mbx62, Center for Open Science.
    2. David Giofrè & Geoff Cumming & Luca Fresc & Ingrid Boedker & Patrizio Tressoldi, 2017. "The influence of journal submission guidelines on authors' reporting of statistics and use of open research practices," PLOS ONE, Public Library of Science, vol. 12(4), pages 1-15, April.
    3. Peter Pütz & Stephan B. Bruns, 2021. "The (Non‐)Significance Of Reporting Errors In Economics: Evidence From Three Top Journals," Journal of Economic Surveys, Wiley Blackwell, vol. 35(1), pages 348-373, February.
    4. Sarah Barakat & Sarah Maguire, 2022. "Accessibility of Psychological Treatments for Bulimia Nervosa: A Review of Efficacy and Engagement in Online Self-Help Treatments," IJERPH, MDPI, vol. 20(1), pages 1-27, December.
    5. Matteo Colombo & Georgi Duev & Michèle B Nuijten & Jan Sprenger, 2018. "Statistical reporting inconsistencies in experimental philosophy," PLOS ONE, Public Library of Science, vol. 13(4), pages 1-12, April.
    6. Coosje L S Veldkamp & Michèle B Nuijten & Linda Dominguez-Alvarez & Marcel A L M van Assen & Jelte M Wicherts, 2014. "Statistical Reporting Errors and Collaboration on Statistical Analyses in Psychological Science," PLOS ONE, Public Library of Science, vol. 9(12), pages 1-19, December.
    7. Colombo, Matteo & Duev, Georgi & Nuijten, M.B. & Sprenger, Jan, 2018. "Statistical reporting inconsistencies in experimental philosophy," Other publications TiSEM 075f5696-ae1a-4aae-9e17-c, Tilburg University, School of Economics and Management.

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