  The ISI subject categories classify journals included in the Science Citation
Index (SCI). The aggregated journal-journal citation matrix contained in the
Journal Citation Reports can be aggregated on the basis of these categories.
This leads to an asymmetrical transaction matrix (citing versus cited) which is
much more densely populated than the underlying matrix at the journal level.
Exploratory factor analysis leads us to opt for a fourteen-factor solution.
This solution can easily be interpreted as the disciplinary structure of
science. The nested maps of science (corresponding to 14 factors, 172
categories, and 6,164 journals) are brought online at
http://www.leydesdorff.net/map06/index.htm. An analysis of interdisciplinary
relations is pursued at three levels of aggregation using the newly added ISI
subject category of "Nanoscience & nanotechnology". The journal level provides
the finer grained perspective. Errors in the attribution of journals to the ISI
subject categories are averaged out so that the factor analysis can reveal the
main structures. The mapping of science can, therefore, be comprehensive at the
level of ISI subject categories.
