  Classification of bibliographic items into subjects and disciplines in large
databases is essential for many quantitative science studies. The Web of
Science classification of journals into ~250 subject categories, which has
served as a basis for many studies, is known to have some fundamental problems
and several practical limitations that may affect the results from such
studies. Here we present an easily reproducible method to perform
reclassification of the Web of Science into existing subject categories and
into 14 broad areas. Our reclassification is at a level of articles, so it
preserves disciplinary differences that may exist among individual articles
published in the same journal. Reclassification also eliminates ambiguous
(multiple) categories that are found for 50% of items, and assigns a
discipline/field category to all articles that come from broad-coverage
journals such as Nature and Science. The correctness of the assigned subject
categories is evaluated manually and is found to be ~95%.
