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Using Effect Size—or Why the P Value Is Not Enough
Open accessAbstract
These statements about the importance of effect sizes were made by two of the most influential statistician-researchers of the past half-century. Yet many submissions to Journal of Graduate Medical Education omit mention of the effect size in quantitative studies while prominently displaying the P value. In this paper, we target readers with little or no statistical background in order to encourage you to improve your comprehension of the relevance of effect size for planning, analyzing, reporting, and understanding education research studies.In medical education research studies that compare different educational interventions, effect size is the magnitude of the difference between groups. The absolute effect size is the difference between the average, or mean, outcomes in two different intervention groups. For example, if an educational intervention resulted in the improvement of subjects' examination scores by an average total of 15 of 50 questions as compared to that of another intervention, the absolute effect size is 15 questions or 3 grade levels (30%) better on the examination. Absolute effect size does not take into account the variability in scores, in that not every subject achieved the average outcome.In another example, residents' self-assessed confidence in performing a procedure improved an average of 0.4 point on a Likert-type scale ranging from 1 to 5, after simulation training. While the absolute effect size in the first example appears clear, the effect size in the second example is less apparent. Is a 0.4 change a lot or trivial? Accounting for variability in the measured improvement may aid in interpreting the magnitude of the change in the second example.Thus, effect size can refer to the raw difference between group means, or absolute effect size, as well as standardized measures of effect, which are calculated to transform the effect to an easily understood scale. Absolute effect size is useful when the variables under study have intrinsic meaning (eg, number of hours of sleep). Calculated indices of effect size are useful when the measurements have no intrinsic meaning, such as numbers on a Likert scale; when studies have used different scales so no direct comparison is possible; or when effect size is examined in the context of variability in the population under study.Calculated effect sizes can also quantitatively compare results from different studies and thus are commonly used in meta-analyses.The effect size is the main finding of a quantitative study. While a P value can inform the reader whether an effect exists, the P value will not reveal the size of the effect. In reporting and interpreting studies, both the substantive significance (effect size) and statistical significance (P value) are essential results to be reported.For this reason, effect sizes should be reported in a paper's Abstract and Results sections. In fact, an estimate of the effect size is often needed before starting the research endeavor, in order to calculate the number of subjects likely to be required to avoid a Type II, or β, error, which is the probability of concluding there is no effect when one actually exists. In other words, you must determine what number of subjects in the study will be sufficient to ensure (to a particular degree of certainty) that the study has acceptable power to support the null hypothesis. That is, if no difference is found between the groups, then this is a true finding.Statistical significance is the probability that the observed difference between two groups is due to chance. If the P value is larger than the alpha level chosen (eg, .05), any observed difference is assumed to be explained by sampling variability. With a sufficiently large sample, a statistical test will almost always demonstrate a significant difference, unless there is no effect whatsoever, that is, when the effect size is exactly zero; yet very small differences, even if significant, are often meaningless. Thus, reporting only the significant P value for an analysis is not adequate for readers to fully understand the results.For example, if a sample size is 10 000, a significant P value is likely to be found even when the difference in outcomes between groups is negligible and may not justify an expensive or time-consuming intervention over another. The level of significance by itself does not predict effect size. Unlike significance tests, effect size is independent of sample size. Statistical significance, on the other hand, depends upon both sample size and effect size. For this reason, P values are considered to be confounded because of their dependence on sample size. Sometimes a statistically significant result means only that a huge sample size was used.3A commonly cited example of this problem is the Physicians Health Study of aspirin to prevent myocardial infarction (MI).4 In more than 22 000 subjects over an average of 5 years, aspirin was associated with a reduction in MI (although not in o
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“Concerning effect size, the r-value was 0.34.This would be counted as medium effect size (Sullivan & Feinn, 2012).”
From Teaching and Learning Science Using YouTube Videos and Discovery Learning in Primary School (2020) · cites this paper as Sullivan & Feinn, 2012
Experimental — accuracy not yet measuredIn the citing paper
“effect sizes, however, are independent of sample size (Sullivan and Feinn, 2012).”
From A chatbot-delivered intervention for optimizing social media use and reducing perceived isolation among rural-living LGBTQ+ youth: Development, acceptability, usability, satisfaction, and utility (2023) · cites this paper as Sullivan and Feinn, 2012
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“These conflicting conclusions demonstrate that relying on statistical significance as the standard for establishing validity is problematic (Sullivan and Feinn 2012), particularly given the small sample sizes typical of most validity studies (usually n \ 100).”
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“We then focus on how the coefficients (i.e., effect size of association) change across model specifications, regardless of statistical significance (Sullivan & Feinn, 2012), to examine the robustness or lack thereof of the associations between PTE types and SU.”
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“Cohen’s d assessed z-score differences at each screening, with effect sizes classified as small (0.2–0.5), medium (0.5–0.8), or large (≥ 0.8) 41 .”
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“The effect size of correlation coefficients was defined as follows: weak ρ = 0.20–0.49, moderate ρ = 0.50–0.80, or strong ρ ≥ 0.80 [79].”
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“The effect sizes are classi fi ed as follows: small ( d = 0.2), medium ( d = 0.5), and large ( d ≥ 0.8) [19].”
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“A possible reason for the lack of observed signi fi cance in the overall population is the small sample size of the cohort, with only 49 participants in the exposure group, which may have limited our ability to detect the differences (Sullivan and Feinn, 2012).”
Rethinking repeat testing: when analytical criteria tell different stories
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“This underscores the role of statistical power and the need to distinguish between statistical significance and clinical relevance when evaluating repeat testing policies [14].”
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“The magnitude of change was evaluated according to Cohen’s criteria (0.8 = large, 0.5 = medium, 0.2 = small, Sullivan and Feinn, 2012).”
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Can social media be a threat or an opportunity to public health via the impacts on diet quality?
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“Small, medium, and large effect sizes are indicated by Cohen’s d values, which are calculated using 0.2, 0.5, and 0.8 thresholds, respectively (17).”
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“…based on the following criteria: d < 0.2 was considered to have no effect, 0.2 ≤ d <0.5 was considered a small effect size, 0.5 ≤ d <0.8 represented a medium effect size, 0.8 ≤ d < 1.3 was regarded a large effect size, and d ≥ 1.3 was regarded as a very large effect size (Sullivan and Feinn 2012 ).”
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“27 For Cohen’s r statistic, |r| ≥ 0.5 is considered a large effect; by this criterion, 12 out of 13 of the increases in confidence were large effects.”
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“The effect size was estimated as small, medium, or large if Cohen’s d was 0.2, 0.5, or 0.8, respectively [36].”
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“Effect size provides a more informative measure of group separation by capturing the strength of the relationship, not just its likelihood [63].”
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“We report effect-size measures using Cramér’s V to quantify the magnitude of differences between cases and controls because large samples can produce statistically significant p -values for trivially minor differences [18].”
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“So, we calculated the effect size to understand the magnitude of the difference between each pair of groups (Sullivan and Feinn, 2012).”
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“ES more than 0.8 is considered as high, 0.5 as medium, and 0.2 is considered as little efficiency (Sullivan and Feinn, 2012).”
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“…spectral deconvolution, missing data input, data normalization (including log transformation) and batch effect correction) (Gromski et al., 2014; Sullivan & Feinn, 2012), and the cleaned data were further processed using various statistical analyses, including t-test, log 2-fold change…”
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“In contrast, the p-value tends to increase in significance probability when the sample size increases (Sullivan & Feinn, 2012).”
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“Including effect size is significant in sustainability research, where the findings may be statistically significant but not necessarily impactful in practice [72].”
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“Hence, we used effect size to interpret the results, providing a more direct picture of the magnitude and practical significance of the effect [28].”
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“The problem with rules of thumb such as these is that they do not consider variability in the data, the effect size of interest or the distribution of the data [10, 12, 13].”
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“Cohen classified effect sizes as small (d = 0.2), medium (d = 0.5), and large (d ≥ 0.8) making our observed effect substantially larger than typical environmental modeling effects [48].”
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“The Vietnamese (β = 0.585, t = 11.950, p 0.001, f 2 = 0.350) and Chinese (β = 0.532, t = 12.152, p 0.001, f 2 = 0.338) subsamples also showed large effect sizes, confirming that celebrity credibility is a strong predictor of brand equity in both cultural contexts.”
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“This is crucial in large datasets where even trivial effects can become statistically significant, as it allows us to focus on the magnitude and practical importance of our findings [20].”
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“Effect size analysis indicated small effects (Cohen’s d < 2; range 0.02–1.08) [31], confirming the clinical reliability of the p values obtained.”
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“Effect-size reporting frameworks 7–9 improve interpretability, yet do not quantify robustness itself.”
Metadata
- Authors
- Sullivan, G. M., & Feinn, R.
- Affiliations
- University of Connecticut (US)
- Corresponding author
- Sullivan, Gail M.
- Venue
- Journal of Graduate Medical Education
- Published
- 2012-09
- Volume
- 4
- Issue
- 3
- Pages
- 279-282
- ISSN
- 1949-8349 (ISSN-L)
- 1949-8357
- PMC
- PMC3444174
- OpenAlex
- W2107026277
- Cited by
- 5,553
- References
- 6
- Citations by year
- 2026: 607 · 2025: 877 · 2024: 759 · 2023: 547 · 2022: 574 · 2021: 499 · 2020: 416 · 2019: 419 · 2018: 320 · 2017: 222 · 2016: 215 · 2015: 83 · 2014: 9 · 2013: 4 · 2012: 2
- FWCI
- 35.75
- Type
- journal article
- Language
- en
- Open access
- bronze
- doi.org
- Locations
- Journal of Graduate Medical Education (publishedVersion)
- Europe PMC (PubMed Central) (publishedVersion)
- PubMed Central (publishedVersion)
- PubMed (publishedVersion)
- Topics
- Social Sciences › Decision Sciences › Statistics, Probability and Uncertainty › Meta-analysis and systematic reviews
- Physical Sciences › Mathematics › Statistics and Probability › Statistical Methods in Epidemiology
- Social Sciences › Education › Educational Research and Methods
- Keywords
- effect size, p-values, statistical significance, substantive significance, standardized mean difference, medical education research, educational intervention, meta-analysis, simulation-based training, resident confidence, test scores, interpretation of study results, graduate medical education, reporting guidelines
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