Statistical Significance and Effect Sizes of Differences among Research Universities at the Level of Nations and Worldwide based on the Leiden Rankings
2019
The Leiden Rankings can be used for grouping research universities by considering universities
which are not significantly different as a homogeneous set. The groups and intergroup relations
can be analyzed and visualized using tools from network analysis. Using the so-called
“excellence indicator” PPtop-10%—the proportion of the top-10% most-highly-cited papers
assigned to a university—we pursue a classification using (i) overlapping stability intervals, (ii)
statistical-significance tests, and (iii) effect sizes of differences among 902 universities in 54
countries; we focus on the UK, Germany, Brazil, and the USA as national examples. Although
the groupings remain largely the same using different statistical significance levels or
overlapping stability intervals, the resulting classifications are uncorrelated with those based on
effect sizes. Effect sizes for the differences between universities are small (w <.2). The more
detailed analysis of universities at the country level suggests that distinctions beyond three or
perhaps four groups of universities (high, middle, low) may not be meaningful. Given similar
institutional incentives, isomorphism within each eco-system of universities should not be
underestimated. For practical purposes, our results suggest that networks based on overlapping
stability intervals can provide a first impression of the relevant groupings among universities.
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