Journal classification metrics and clustering algorithms?

[Jacques Wainer]

I am interested in the issue of Journal Classification used by Scopus,
Thomson Reuters, Google Scholar and so on. I do not know if this is
strategic information for the companies, but I would appreciate if
anyone could point me out to references on:

a) which metric the companies use to measure the similarity between journals?

b) which clustering/community detection algorithm they use?

Regrading the metric (a) I assume that it is bibliographic coupling,
but there are two subquestions:

a1) bibliographic coupling is, in principle, an asymmetric measure -
that is, the similarity from A to B may not be the same as the
similarity from B to A. If journal A cites 20 documents in common to B
but A cites in total 200 documents while B cites 2000 documents, the
two similarities are one order of magnitude different! Do they use any
symetrization procedure?

a2) Is there a fixed interval for collecting the citations made by a
journal (say citations in the last two years?) or should one use the
whole history of the journal?


Thank you

jacques wainer