You should not depend solely on citation data in your journal evaluations. Citation data are not meant to replace informed peer review. Careful attention should be paid to the many conditions that can influence citation rates such as language, journal history and format, publication schedule, and subject specialty.
You should also consider the following four conditions, which may affect journal's ranking and Impact Factor:
Most of the journals in Thomson ISI are English-language from North America, Western Europe and Australia.
Review journals and original research journals are treated the same even though the citation patterns of these types of articles differ.
The Journal Citation Reports (JCR) database is a unique and comprehensive tool providing bibliometric analysis of more than 5,900 journals in the areas of science and technology and of 1,700 journals in the social sciences. Coverage is international and includes journals published by over 3,300 publishers in 60 countries.
The aim of the JCR is to provide a systematic and objective means of determining the relative importance of science and social sciences journals within their subject categories. Information for each title includes the "impact factor" (measurement of the frequency with which the average article has been cited in a particular year) and the "immediacy index" (how quickly the average article in a journal is cited). The JCR can show you the:
JCR uses Thomson Reuters (ISI Web of Knowledge) citation data.