By the beginning of the twentieth century, the volume of
primary publications had reached such a large size that it began
to be difficult for scientists to find information. In order to
simplify literature searching secondary publications began to be
produced. These can take the form of single volumes, such as
handbooks, or can be published at regular intervals, such as
abstracts and indexes.
Well known examples of other types of secondary publications are
encyclopaedias and yearbooks. These are often grouped under the
heading "reference literature" in libraries.
In primary publications, information on a given topic is widely
scattered, for example, in many different journals. Secondary
publications are usually subject-oriented. Information from many
primary sources is collected together and organised in a
structured form, for example, under subject headings, and
designed to facilitate information searching. Indexes provide
search entry points, for example, to authors or subjects, and
primary publications can be identified by means of detailed
information as to the author, source of the publication and year
of publication. Indexes only contain titles of the primary
publications (e.g. articles) together with identification details,
such as author(s), name of journal, date of publication, volume,
issue number and pagination.
A different type of secondary publication is the Citation Index.
In every field of scholarship, research workers and practitioners
cite references to earlier publications related to the work
described in their own papers. Through these references (citations)
an author expresses subject relationships between the current
article and the cited references. A citation index is based on
these relationships. It lists publications that have been cited
and identifies the sources of these citations. Starting from a
known relevant reference, it is possible to trace subsequent
articles which refer to the original document, thus leading to
more recent literature than the first known item. Citation
indexes are designed to facilitate searching forwards in time
from a known relevant paper.
These Citation Indexes are divided into three main parts: