Table 1. Interdisciplinarity Measures.

Measure	Description
Ratio of references to other subject categories
Number of referenced subject categories	n
1—Gini	$1-\frac{{\displaystyle \sum \left(2i-n-1\right)x_i}}{n{\displaystyle \sum x_i}}$, where i is the index, x i is the number of references to the i-th subject category, and subject categories are sorted by x i in non-decreasing order.
Simpson index	$1-{\displaystyle \sum p_i^2}$, where p i = x i/X, and X = ∑x i
Shannon entropy	− ∑p i log(p i)
Average dissimilarity between referenced subject categories	$\frac{1}{n\left(n-1\right)}{\displaystyle \sum _{i\ne j}{d}_{ij}}$, where d ij is the dissimilarity between subject category i and j. Specifically, d ij = 1 − s ij, where s ij is the cosine similarity between subject category i and j based on their co-citation matrix.
Rao-Stirling diversity	${\displaystyle \sum _{i\ne j}{p}_i{p}_j{d}_{ij}}$