  Policy makers and managers sometimes assess the share of research produced by
a group (country, department, institution). This takes the form of the
percentage of publications in a journal, field or broad area that has been
published by the group. This quantity is affected by essentially random
influences that obscure underlying changes over time and differences between
groups. A model of research production is needed to help identify whether
differences between two shares indicate underlying differences. This article
introduces a simple production model for indicators that report the share of
the world's output in a journal or subject category, assuming that every new
article has the same probability to be authored by a given group. With this
assumption, confidence limits can be calculated for the underlying production
capability (i.e., probability to publish). The results of a time series
analysis of national contributions to 36 large monodisciplinary journals
1996-2016 are broadly consistent with this hypothesis. Follow up tests of
countries and institutions in 26 Scopus subject categories support the
conclusions but highlight the importance of ensuring consistent subject
category coverage.
