Eugene Garfield first proposed the idea of journal impact factors in 1955 to measure the impact and influence of academic journals. In the 1960s, the Science Citation Index was developed to track citations between papers. Starting in 1975, the Journal Citation Reports used Web of Science data to annually rank journals within disciplines based on their impact factors, calculated as the citations in the current year to articles published in the previous two years divided by the total number of citable items published in those two years. While impact factors provide a metric for comparing journals within a field, they should not be used to compare journals across different disciplines due to variability in citation conventions between fields.