Using the correct terminology helps to ensure that the same words are used for the same concepts, which is crucial for a clear communication and for avoiding misunderstandings. The term 'nonparametric data' appears often in the statistics section of research reports. For example, Dugan et al. (1) state that the "data from patients in the two groups were compared using Mann-Whitney U tests for nonparametric data."
However, statistical tests are used to evaluate sampling uncertainty, to test a hypothesis about the properties of an unobservable population represented by a sample of studied patients. The hypothesis may or may not be based on an assumption about a specific data distribution (e.g. Normal) for a test of its parameters. If this is the case, the hypothesis is parametric. Such hypotheses are tested using asymptotic methods. Otherwise, the hypothesis is nonparametric, which requires a distribution-free method. "Nonparametric data" is a nonexistent concept.
References
1. Dugan B, Welch CA, Rahman A, Korona MV, Deipolyi AR. Reduction of hemorrhagic complications after non-focal renal biopsy with pre-procedure desmopressin administration. Diagn Interv Radiol. 2026 Mar 23. doi: 10.4274/dir.2026.263734. Epub ahead of print. PMID: 41866967.
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