For example, Journal A has an impact factor of 4.327, and Journal B has an impact factor of 1.045. Is Journal A "better" than Journal B? You could conceivably make that argument, if you first accept the notion that quality equates with citedness, AND if journals A and B are both in the same field. But if A is in Biochemistry, and B is in Clinical Pharmacy, no such judgment can be made, as citation behavior varies considerably from field to field.
Impact factor can also vary based on the number and types of articles a journal publishes. Review articles tend to be more heavily cited than full papers or communications, so journals and annuals that publish mostly reviews will often have high impact factors. Journals that publish only a few articles in a given year may also have disproportionately high impact factors. Similarly, one very highly cited paper can skew a journal's impact factor.
Impact factors for journals covered by ISI are published annually in Journal Citation Reports. All WOS Source Journals are ranked within one or more relevant subject categories, such as CHEMISTRY, ORGANIC or SPECTROSCOPY. You can also compile customized lists. JCR also contains data on historical trends, immediacy index, cited half-life, etc.
While Impact Factors are useful within certain limits, they are subject to manipulation. Critics point to a lack of transparency and reproducibility as an ongoing problem. Nor does a journal's IF relate in any way to the impact or quality of an individual author or paper. As a result, the use of journal impact factors in personnel and funding decisions is strongly discouraged.
SJR SCImago is different journal ranking system based on citation data from Elsevier's Scopus database. Another project called Eigenfactor uses network theory to group journals and their citations and evaluate their relative importance.