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The formula for standardization is as follows: (1) (2) Here, <i>x_ij</i> denotes the raw value of the <i>j</i>-th indicator in the <i>i</i>-th year, while <i>y_ij</i> represents the standardized index value. Note that in Eqs (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0317180#pone.0317180.e001" target="_blank">1</a>) and (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0317180#pone.0317180.e002" target="_blank">2</a>), the terms “positive” and “negative” within the parentheses denote the standardization methods for positive and negative indicators, respectively. This section provides a detailed explanation of the principles behind the data normalization used in this study. (PDF)