A note on the Wilcoxon-Mann-Whitney test and tied observations

Recently, it was recommended to omit tied observations before applying the two-sample Wilcoxon-Mann-Whitney test McGee M. et al. (2018). Using a simulation study, we argue for exact tests using all the data (including tied values) as a preferable approach. Exact tests, with tied observations include...

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Bibliographic Details
Published in:PloS one 2024-08, Vol.19 (8), p.e0309074
Main Authors: Neuhäuser, Markus, Ruxton, Graeme D
Format: Article
Language:English
Subjects:
Analysis
Approximation
Computer and Information Sciences
Computer Simulation
Data Interpretation, Statistical
Engineering and Technology
Error analysis
Humans
Mann-Whitney U test
Methods
Models, Statistical
Permutations
Physical Sciences
Resampling (Statistics)
Research and Analysis Methods
Science Policy
Simulation
Software
Statistics, Nonparametric
Online Access:Get full text
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Summary:Recently, it was recommended to omit tied observations before applying the two-sample Wilcoxon-Mann-Whitney test McGee M. et al. (2018). Using a simulation study, we argue for exact tests using all the data (including tied values) as a preferable approach. Exact tests, with tied observations included guarantee the type I error rate with a better exploitation of the significance level and a larger power than the corresponding tests after the omission of tied observations. The omission of ties can produce a considerable change in the shape of the sample, and so can violate underlying test assumptions. Thus, on both theoretical and practical grounds, the recommendation to omit tied values cannot be supported, relative to analysing the whole data set in the same way whether or not ties occur, preferably with an exact permutation test.
ISSN:1932-6203
1932-6203
DOI:10.1371/journal.pone.0309074