Following one of our publications last year, the data from this recent study are perfect to show how much standardization is important. In the below example, the symbols represent the % differences in several variables between 2 groups of soccer players (more vs. less mature). The numbers above each symbol refer to the standardized differences (effect size), and the stars, to the chances for the differences to be substantial (i.e., clearly greater than the practically important effect, the smallest worthwhile change). Read more here about these key concepts.
What stands out is the lack of association between the % differences and their actual magnitude (i.e., standardized difference). This is particularly evident when comparing the differences in high speed activities (HSA, >45%, stdz diff +0.7) and height (5%, but stdz diff +1.6). In accordance with the standardized differences (and not the %), the chances for the differences in height are greater than for HSA ! Conclusion: don’t trust percentages. Report magnitudes and chances for the differences to be real.