The time required to referee manuscripts may also affect impact. If manuscript processing is delayed, references to articles that are no longer within the JCR two-year window will not be counted. Alternatively, the appearance of articles on the same subject in the same issue of a journal may have an upward effect. Opthof showed how journal impact performance can vary from issue to issue. For greater precision, it is preferable to conduct item-by-item journal audits so that any differences in impact for different types of editorial items can be taken into account. Other objections to impact factors are related to the system used in JCR to categorize journals. In a perfect system it ought to be possible to compare journals with an identical profile. But in fact there rarely are two journals with identical semantic or bibliographic profiles. ISI’s heuristic, somewhat subjective methods for categorizing journals are by no means perfect, even though their specialists do use citation analysis to support their decisions. Some might argue that JCR categories are larger than necessary. Recent work by Alexander Pudovkin and myself is an attempt to group journals more objectively. We rely on the two-way citational relationships between journals to reduce the subjective influence of journal titles. Three decades ago, I demonstrated that journal titles can be deceiving. Citation analysis proved the Journal of Experimental Medicine was a leading immunology journal. It still is one of the five top immunology journals based on its impact factor.