Impact Factor

Impact Factor shows the average number of annual citations per article in a journal, rounded to two decimal places. Impact Factor rankings should be considered with caution, as they are biased against journals that publish a larger number of shorter articles, such as book reviews. Nevertheless, if two law journals have a similar composition of articles, notes, and book reviews, then from an author's viewpoint it is reasonable to compare the Impact Factor of each to see which is a better journal with which to publish. The implication of a similar ranking between two journals by Total Cites but a dissimilar ranking by Impact Factor is that the journal ranked lower by Impact Factor is publishing some articles of lesser citable quality or of less general interest. The Combined Score ranking (a weighting of both Impact Factor and Total Cites) thus offers a more balanced and inclusive journal ranking.

Impact Factor is calculated by first searching Westlaw for citing articles in five separate yearly segments (previously eight for 2003 to 2017), using the same search statement for each segment, except that the DATE ADDED field is changed in each search: AD(2014), AD(2015), ..., AD(2018). The number of citing articles from each yearly segment of additions to Westlaw is then divided by the number of items published in the journal in that same yearly segment. For example, if the survey period is 2014-2018, then the yearly segment of 2014 articles added to Westlaw will be citing 2014 articles; the yearly segment of 2015 articles will be citing 2014 and 2015 articles, etc., until we reach the 2018 yearly segment which will be citing articles from 2014 through 2018. Assuming for example that a journal consistently publishes 20 items each year, and that the number of citing articles from the 2015 segment is 30, its 2015 Impact Factor is 30/40=0.75. If the number of citing articles in the 2018 segment is 150, then that year's Impact Factor is 150/100=1.5. Then, in order to discard less representative outliers, the median of those values is recorded as the journal's Impact Factor (usually for a five-year publication range this will be the average of the two Impact Factors closest to mid-range). In other words, Impact Factor is the median of the journal's annual Impact Factors, and those annual Impact Factors are calculated by dividing the number of citing articles added to the Westlaw JLR database in that year by the number of items published by the journal in that year and any other year back to the beginning of the survey period.

The basic methodological challenge in calculating Impact Factor is determining the number of articles published by each journal for the date period; there is no completely uniform, satisfactory, and automated method for doing this. For calculation purposes, "Articles" includes entries such as notes, comments, book reviews, letters, and attributed introductions and forewords, as well as formal articles. For 2019 onward, article-quantity data is obtained from Hein Online’s Law Journal Library database, cross-referenced with other commercial databases and journals’ own websites. Prior to 2019, most article-quantity data was gathered using EBSCO’s Index to Legal Periodicals (ILP) database and Legal Source database.

If a journal’s output is not adequately indexed in Hein Online, then the journal’s number of published articles is retrieved from other sources, typically the journal’s own website, Westlaw, or LexisNexis. A “manual” count is often necessary, examining each issue’s tables of contents for the years needed. For journals where indexing is not available and a manual count is not feasible, an estimate is made from information known about the number of articles published in past years and the journal’s publication schedule. For 2010-2017 and previous survey years, the estimated article counts were calculated by use of a weighted average of article counts over the preceding four years. The first preceding year is weighted at 40%, second preceding year at 30%, third preceding year at 20%, and fourth preceding year at 10%. Data for the 2018 and 2019 survey years was gathered late enough after the end of the calendar years to obtain accurate information for actual article numbers published by the journals in those years, thus avoiding the article-count approximation that was necessary in previous Rankings releases.