Publishing metrics

The range of models used to measure the impact of journals and articles is constantly increasing, though most are based on the level of citations. As a signatory to DORA, the Royal Society offers a variety of journal and article-based metrics.

Impact factor

The impact factor is a measure of how often an average article in a journal has been cited. It is calculated by dividing the number of current year citations to source items published in that journal during the previous 2 years.

While the impact factor is a relatively easy measure to calculate and understand, it does have some limitations. Given the highly skewed nature of the citation distribution of a journal's articles, the appropriateness of using a mean has been questioned. Also, most journals contain a significant proportion of articles that are never cited. Such articles can be seen as ‘benefiting’ unfairly from the impact factor of the journal in which they appear.

• 5 year impact factor: is the impact factor calculated using a base of 5 years’ worth of cited articles, rather than 2. This gives a fairer picture of journals in fields with slower citation patterns, such as mathematics.
• Cited half-life: the number of years, counting back from the current year, which account for half the total citations received by the cited journal in the current year. This provides a measure of how ‘long-lived’ the articles are in terms of their influence on the literature.
• Immediacy index: the average number of times that an article is cited in the same year it is published. It gives an indication of how topical the material in the journal is.

Eigenfactor

The Eigenfactor gives an indication of the overall contribution of the journal, as a whole, to the literature.

It does this using an algorithm similar to that of Google's ‘PageRank’ to count citations into and out of the journal and to weight them according to the ranking of the source or destination. It uses the Thomson Reuters journal dataset and it is intended to measure how likely a journal is to be used or the amount of time a reader will spend reading it. Large journals rank more highly in the Eigenfactor system than small journals (in contrast to the impact factor, which is independent of size).

• Article influence: measures the relative importance of the journal on a per-article basis. It is the journal's Eigenfactor Score divided by the fraction of articles published by the journal. That fraction is normalised so that the sum total of articles from all journals is 1. 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.

Article usage metrics

We collect article usage statistics on a daily basis and display them on each article within the ‘Info & Metrics’ tab. The statistics comprise the number of times an article abstract, PDF and full-text view is loaded by a user from individual journal websites.

Note that all articles have zero usage upon publication and the latest available metrics will be displayed 24 hours after they have been collected.

Altmetrics

Altmetrics are non-traditional metrics proposed as an alternative to citation impact metrics.

We use a partner organisation called Altmetric to track and display the online activity around an article. Altmetric track social media sites, newspapers and magazines for mentions of each published article. The aim is to help authors quantify the attention their article is receiving and to help readers establish the articles their peers think are interesting.

The Altmetric ‘doughnut’ can be found on the ‘Info & Metrics’ tab of each article.