# Measuring a journal’s impact

## Scopus journal metrics

Understanding the impact of a journal can help you decide whether to submit your article.

Impact per Publication (IPP) is comparable to the Impact Factor, but uses a citation window of three years, which is considered to be the optimal time period to accurately measure citations in most subject fields. Taking into account the same peer-reviewed scholarly papers in both the numerator and denominator of the equation provides a fair impact measurement of the journal and diminishes the chance of manipulation. IPP measures the ratio of citations in a year (Y) to scholarly papers published in the three previous years (Y-1, Y-2, Y-3) divided by the number of scholarly papers published in those same years (Y-1, Y-2, Y-3).

Source Normalized Impact per Paper (SNIP) provides a novel bibliometric perspective, correcting for subject-specific characteristics of the field someone is publishing in. This means that, unlike the Impact Factor, SNIP numbers can be compared for any two journals, regardless of the field they are in. SNIP is defined as the ratio of the raw Impact per Publication divided by the Relative Database Citation Potential. The raw Impact per Publication is the same as IPP. The resulting ratio is then divided by the Relative Database Citation Potential.

SCImago Journal Rank (SJR) is based on citation data of the more than 20,000 peer-reviewed journals indexed by Scopus from 1996 onwards. Citations are weighted, depending on the rank of the citing journal: A citation from an important journal will count as more than one citation; a citation coming from a less important journal will count as less than one citation. The SJR of journal J in year X is the number of weighted citations received by J in X to any item published in J in (X-1), (X-2) or (X-3), divided by the total number of articles and reviews published in (X-1), (X-2) or (X-3).

The h-index is a metric for evaluating individual scientists – it rates a scientist's performance based on their career publications, as measured by the lifetime number of citations each article receives. The measurement is dependent on both quantity (number of publications) and quality (number of citations) of an academic's publications. If you list all of a scientist's publications in descending order of the number of citations received to date, their h-index is the highest number of their articles, h, that have each received at least h citations. So, their h-index is 10 if 10 articles have each received at least 10 citations; their h-index is 81 if 81 articles have each received at least 81 citations.

The journal Impact Factor is published every year by Thomson Reuters. It measures the number of times an average paper in a particular journal has been referred to.

The Impact Factor of journal J in the calendar year X is the number of citations received by J in X to any item published in J in (X-1) or (X-2), divided by the number of source items published in J in (X-1) or (X-2). The Impact Factor can be a useful way of comparing citability of journals, but the absolute Impact Factor is of limited use without those of other journals in the field against which to judge it. You can find the most recent Impact Factors of our individual journals on their homepages.

The Five-year Impact Factor is similar in nature to the regular 'two-year' Impact Factor, but instead of counting citations in a given year to the previous two years and dividing by source items in these years, citations are counted in a given year to the previous five years and again divided by the source items published in the previous five years. While still showing changes over time, the five-year measure presents a much smoother variation.

## Other journal metrics

Immediacy Index: Published by Thomson Reuters, the Immediacy Index is a measure of the speed at which content in a particular journal is picked up and referred to. The Immediacy Index of journal J in the calendar year X is the number of citations received by J in X to any item published in J in X, divided by the number of source items published in J in X.

Cited Half-Life: Published by Thomson Reuters, the Cited Half-Life is a measure of the 'archivability' of content in a particular journal, or of how long content is referred to after publication. The Cited Half-Life of journal J in year X is the number of years after which 50% of the lifetime citations of J's content published in X have been received.

The Eigenfactor and Article Influence are based on data held in Thomson Reuters' Journal Citation Reports. The Eigenfactor of journal J in year X is defined as the percentage of weighted citations received by J in X to any item published in (X-1), (X-2), (X-3), (X-4), or (X-5), out of the total citations received by all journals in the dataset. Only citations received from a journal other than J are counted. Article Influence is calculated by dividing the Eigenfactor by the percentage of all articles recorded in the Journal Citation Reports that were published in J. Article Influence is therefore conceptually similar to the Impact Factor and SCImago Journal Rank.