Applies to: Primarily journal articles, but also other kinds of research outputs, such as book chapters and conference proceedings that are sufficiently covered by abstract and citation databases.

The Field Normalized Citation Impact (FNCI) is the ratio between the actual citations received by a publication and the average number of citations received by all other similar publications. The latter is referred to as the expected number of citations. Similar publications are ones in the same subject category, of the same type (i.e. article, review, book chapter, etc.), and of the same age (i.e. publication year).

Metric calculation: A FNCI is measured by dividing the number of citations a publication received by the average number of citations to publications in a database published in the same year, of the same type, and within the same subject category. When multiple publications are being considered, the ratio between the actual and average citations for each publication are calculated first. A typical indicator is the Mean Normalized Citation Score (MNCS), which is the mean result of all FNCI of all publications included in the analysis. Publications can also be assigned to more than one subject category. In these cases, usually the publication and its citation counts are proportionally distributed across the relevant subject categories.

SOURCE: https://www.metrics-toolkit.org/metrics/field_weighted_citation_impact/