The growth rate of scientific publication has been studied from 1907 to 2007 using available data from a number of literature
databases, including Science Citation Index (SCI) and Social Sciences Citation Index (SSCI). Traditional scientific publishing,
that is publication in peer-reviewed journals, is still increasing although there are big differences between fields. There
are no indications that the growth rate has decreased in the last 50 years. At the same time publication using new channels,
for example conference proceedings, open archives and home pages, is growing fast. The growth rate for SCI up to 2007 is smaller
than for comparable databases. This means that SCI was covering a decreasing part of the traditional scientific literature.
There are also clear indications that the coverage by SCI is especially low in some of the scientific areas with the highest
growth rate, including computer science and engineering sciences. The role of conference proceedings, open access archives
and publications published on the net is increasing, especially in scientific fields with high growth rates, but this has
only partially been reflected in the databases. The new publication channels challenge the use of the big databases in measurements
of scientific productivity or output and of the growth rate of science. Because of the declining coverage and this challenge
it is problematic that SCI has been used and is used as the dominant source for science indicators based on publication and
citation numbers. The limited data available for social sciences show that the growth rate in SSCI was remarkably low and
indicate that the coverage by SSCI was declining over time. National Science Indicators from Thomson Reuters is based solely
on SCI, SSCI and Arts and Humanities Citation Index (AHCI). Therefore the declining coverage of the citation databases problematizes
the use of this source.