A description of the terminology and methodology used in this supplement, and a guide to the functionality available free online at natureindex.com
The Nature Index is a database of author affiliations and institutional relationships. The index tracks contributions to articles published in a group of highly selective science journals, chosen by an independent group of active researchers.
The Nature Index provides absolute and fractional counts of publication productivity at the institutional and national level and, as such, is one indicator of global high-quality research output. Data in the Nature Index are updated regularly, with the most recent 12 months made available under a Creative Commons licence at natureindex.com. The database is compiled by Springer Nature.
The list of journals tracked by the Nature Index will be extended in 2018.
Nature Index metrics
There are several measures provided by the Nature Index to track affiliation data. The simplest is the article count (AC). A country or institution is given an AC of 1 for each article that has at least one author from that country or institution. This is the case regardless of the number of authors an article has, and it means that the same article can contribute to the AC of multiple countries or institutions.
To get a sense of a country's or institution's contribution to an article, and to ensure they are not counted more than once, the Nature Index uses the fractional count (FC), which takes into account the share of authorship on each article.
The total FC available per paper is 1, which is shared among all authors under the assumption that each contributed equally. For instance, a paper with 10 authors means that each author receives an FC of 0.1. For authors who have joint affiliations, the individual FC is then split equally between each affiliation.
Another measure used is the weighted fractional count (WFC), which applies a weighting to the FC to adjust for the over-representation of papers in astronomy and astrophysics. Four journals tracked by the index in these disciplines publish about 50% of all papers in journals in the physical sciences — approximately five times the equivalent percentage for other fields. Therefore, although the data for astronomy and astrophysics are compiled in the same way as for all other disciplines, articles from these journals are assigned one-fifth the weight of other articles (the FC is multiplied by 0.2 to derive the WFC).
The total FC or WFC for an institution is calculated by summing the FC or WFC for individual authors. The process is similar for countries, although complicated by the fact that some institutions have overseas labs that will be counted towards host country totals.boxed-text
The fourth metric is bilateral collaboration score (CS). A bilateral collaboration can be between any two institutions or countries co-authoring at least one paper in the journals tracked by the Nature Index. CS is derived by summing the FCs from papers with authors from both institutions. If institution A has co-authored a paper with another institution B, then the collaboration score between A and B is the sum of the FC for A+B.
Every effort is made to count affiliations consistently, with a background of reasonable assumptions. For more on how the affiliation information is processed and counted, see the FAQ section at natureindex.com.
Nature Index 2017 United States is based on data from natureindex.com, covering articles published during five years from 1 January 2012 to 31 December 2016.
The tables in this supplement rank institutions by WFC, as it provides a more even basis for comparison across multiple disciplines, and in determining the relative contribution of each institution. Each state's WFC is calculated by summing that of all of its principal institutions. This issue also includes data and analyses from the Web of Science.