Not all journals have impact factors.
Citation data only for the records indexed by JCR.
Limited number of small-sized journals.
Limited number of journals in non-English languages.
Very few open access journals.
New journals do not have Journal Impact Factors.
Journal impact factors may vary between different academic disciplines.
Impact factor depends on the journal's size.
Impact factor depends only on the articles that are cited within two years after publication.
Self-citation can distort low journal impact factors.
Journals with a large proportion of non-citable items (such as editorials) can have lower journal impact factors. Reviewd articles are cited more often and can change results.