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The study by Doudin et al. [2], exploring the association between serum 25(OH)D and hematological indices in adolescents, stands out for the wise use of cross-sectional data from a German cohort to explore a novel issue related to vitamin D.
The authors’ interpretation of the findings is misled by the use of p value. Applying it to assess associations whose underlying plausibility is ignored and is not suggested by the correlation coefficients, easily leads to false-positive findings. In fact, p values are strongly related to sample size [6], “sometimes attracting attention to very small effects that have little real-world importance” [1].
The authors acknowledge that “the correlation coefficients and the effect sizes are very small” [2]. Indeed, the correlation coefficient estimates range from r = −0.04 to r = 0.08 (very close to r = 0, i.e., absence of correlation), and the differences on hematological indices by levels of serum 25(OH)D are minimal (e.g., Hb [g/dl] 1st tertile: 13.7 ± 1.2; 2nd tertile: 13.5 ± 1.1; 3rd tertile 13.6 ± 1.1), in spite of the low estimated p values. None of the “statistically significant” differences found have a clear “clinical relevance” [3].
This frequent methodological error prompted methodologists and editors to discourage the widespread use of p values and to promote the use of alternative measures, with scarce success [4]. “False positive findings are detrimental to science and society, as once published, they accumulate persistent untrue evidence” [5].
Abbreviations
- 25(OH)D:
-
25-hydroxyvitamin D
References
Boyd JC, Annesley TM (2014) To P or not to P: that is the question. Clin Chem 60:909–910
Doudin A, Becker A, Rothenberger A, Meyer T (2018) Relationship between serum 25-hydroxyvitamin D and red blood cell indices in German adolescents. Eur J Pediatr 177:583–591
Goodman S (2008) A dirty dozen: twelve p-value misconceptions. Semin Hematol 45:135–140
Ludwig DA (2005) Use and misuse of p-values in designed and observational studies: guide for researchers and reviewers. Aviat Space Environ Med 76:675–680
Lytsy P (2017) Creating falseness - how to establish statistical evidence of the untrue. J Eval Clin Pract 23:923–927
Wellek S (2017) A critical evaluation of the current “p-value controversy”. Biom J 59:854–872
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Daniel Virella: critical reading of the article by Doudin et al., review of the statistical analysis, review of the literature, and writing this manuscript.
Ana Luisa Papoila: critical reading of the article by Doudin et al., review of the statistical analysis, and writing this manuscript.
Luis Pereira-da-Silva: critical reading of the article by Doudin et al., and contributed to the final manuscript.
All authors read and approved the final manuscript.
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The authors declare that they do not have a financial relationship with the organization that sponsored the research.
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Communicated by Mario Bianchetti
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Virella, D., Papoila, A.L. & Pereira-da-Silva, L. Association between serum 25(OH)D and hematological markers of erythropoiesis: the curse of large numbers, the treachery of p value. Eur J Pediatr 178, 429 (2019). https://doi.org/10.1007/s00431-018-3296-6
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DOI: https://doi.org/10.1007/s00431-018-3296-6