Like the Impact Factor, the Eigenfactor® Score and Article Influence® Score use citation data to assess and track the influence of a journal in relation to other journals. The Eigenfactor Score calculation is based on the number of times articles from the journal published in the past five years have been cited in the JCR year, but it also considers which journals have contributed these citations so that highly cited journals will influence the network more than lesser cited journals.
The Article Influence determines the average influence of a journal's articles over the first five years after publication. It is calculated by dividing a journal’s Eigenfactor Score by the number of articles in the journal, normalized as a fraction of all articles in all publications. The mean Article Influence Score is 1.00. A score greater than 1.00 indicates that each article in the journal has above-average influence. A score less than 1.00 indicates that each article in the journal has below-average influence.
Many other methods of measuring the impact of published articles involve metrics computed at the article or author level rather than the journal level.
For a fuller explanation of those methods, see our guide to Assessing Article and Author Influence.
PLoS is one example of the use of article-level metrics.
PLoS Article-Level Metrics measure the dissemination and reach of published research articles.
Traditionally, the impact of research articles has been measured by the publication journal. But a more informative view is one that examines the overall performance and reach of the articles themselves. Article-Level Metrics are a comprehensive set of impact indicators that enable numerous ways to assess and navigate research most relevant to the field itself, including:
Article-Level Metrics are available, upon publication, for every article published by PLOS.
The main difference between SJR and the Impact Factor is that the IF gives equal weight to all citations, making no distinction between citations published in some obscure journals and citations published in Nature or Lancet. To address this issue, SJR uses mathematical approach behind the Google's PageRank algorithm and adapts it to journal metrics. The PageRank model type weights citations from journals according to how highly cited the journal itself is.
Apart from assigning different values to citations depending on the importance of the journals, there are other essential differences between IF and JCR.
- breadth of SJR's scientific, technical and medical journal coverage. SJR makes use of data supplied by Scopus which covers almost 20,000 journals, including many STM journals not tracked by Thomson Scientific. Thomson Scientific often waits several years before including new journals in, and in some cases may not track some journals at all, even though they are highly cited. Scopus has a more systematic policy on content inclusion, including OA journals.
- impact factors are derived from citations in a single year to articles from the two preceding years, the SJR looks at citations made in a three year period, of articles published in an earlier, but overlapping, three year period. This makes the SJR a more stable indicator of trends than impact factors, which often fluctuate
substantially from year to year.