Article output is counted in three ways:
- Article count (AC): where a count of one is assigned to an institution or country if one or more authors of the research article is from that institution or country, regardless of how many co-authors there are from outside that institution or country.
- Fractional count (FC): that takes into account the percentage of authors from that institution (or country) and the number of affiliated institutions per article. For calculation of the FC, all authors are factored to have contributed equally to the article. The maximum combined FC for any article is 1.0.
- Weighted fractional count (WFC): a modified version of FC in which fractional counts for articles from specialist astronomy and astrophysics journals have been down weighted. These journals encompass a much larger proportion of the total publication output of these fields than any other field covered by the Nature Index. So the WFC allows ordering of institutions and countries so as not to give undue emphasis to these fields. The weighting is achieved by multiplying the fractional count from these astronomy and astrophysics journals by a factor of 0.2. This down weighting is in proportion to an approximation of the level to which astronomy and astrophysics articles are overrepresented compared to the total publication output of other fields covered by the Nature Index.
The ratio of AC to FC gives an indication of the degree to which an institution or country collaborates in its research. Broadly speaking, if the AC is much higher than the FC it indicates a high degree of external collaboration and dependency on external resources. If the AC is close in value to the FC it indicates limited collaboration with external researchers and a strong dependency on internal resources.