Journals can also be classified by ISI quartile. Here, ISI quartiles of impact are calculated by averaging the ISI annual impact factor (JCR) of a journal over the last five years, and then ranking the journals within their ISI subject category. Hence, a journal falls into the category of “ISI 1st“ if its average impact factor ranks in the top 25% of its subject category. If multiple subject categories apply to a journal, the most applicable subject category was selected in these calculations.