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 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 monthly, with the most recent 12 months of data 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 is under review, and from 2016 will be extended to include the clinical sciences.
Nature Index metrics
There are four 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 whether an article has one or a hundred authors, and it means that the same article can contribute to the AC of multiple countries or institutions.
To get a sense of a country or institution's contribution to an article, and to remove the possibility of counting articles more than once, the Nature Index uses the fractional count (FC), which takes into account the relative contribution of each author to an article. The total FC available per paper is 1, which is shared between 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.
The third 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. The four journals in these disciplines publish about 50% of all papers in international journals in this field — 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 (i.e., 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 their host country totals. What's more, there is great variability in the way authors present their affiliations. Every effort is made to count affiliations consistently, with a background of reasonable assumptions.
The fourth metric is collaboration score. When evaluating the collaborative effort of an individual institution or country, an average collaboration score is derived. This is the average of the FCs for all the bilateral relationships for that institution or country. If institution A has relationships with two others, B and C, then the collaboration score is average of the FC for A + B and A + C.
When assessing the collaborative effort between two institutions, or two countries, a bilateral collaboration score can be measured. This is the sum of the FCs from papers with authors from both institutions.
For more information on how the affiliation information is processed and counted, please see the FAQ section at natureindex.com.
Nature Index 2016 Collaborations is based on data from natureindex.com, covering articles published between 1 January 2012 and 31 December 2015. In most analyses, large conglomerates, such as CNRS in France, have been separated into their constituent parts to give a more detailed picture of collaborations.