Journal rankings try to paint a picture of the quality of a journal. They give journals a level of prestige that can make them highly competitive and leads to a lot of people watching these journals. With more eyes on a journal, people are more likely to see articles in these titles and, most significantly, tend to use these materials more often in their own research.
Each system has its own method of calculating its scores. However, journal rankings are based on how often articles in these journals get cited and are typically averaged based on the number of articles published in the journal. Sometimes there can be weighting based on source of the citation (sort of what Google does with links).
Great, Journals are ranked, but what does this mean for me?
Well, there are a few points to consider when looking at journal rankings.
1. Higher ranked journals give articles published within them a better chance of being seen. These obviously do not take into account circulation stats. In this case, seen relates more to the attention paid to the higher ranking journals because they're considered important. Closer attention means other researchers are more likely to consider your research in light of their own. Higher ranked journals are also more likely to be indexed in multiple databases, which means that there are additional ways that people will come to your work if it's published there.
2. Publishing in Higher ranked journals comes with a certain level of prestige. Usually it's pretty competitive to get into a journal that is ranked so highly. Many reasearchers set getting published in a top ranked journal as a goal for 'one day'
3. Highly ranked journals are great sources as well. Academic discourse is a two way street, and we're all pressed for time
There are limitations to such rankings and they're important to note.
1. These rankings are often dependent on the provider. If a journal isn't covered in the database providing the ranking information, then it's not going to be ranked.
2. The rankings are of journals. They provide an idea of how often articles, on average, are cited in a journal (and therefore their impact on academia). However, it doesn't speak to how successful and individual author/article is.
3. Education is tricky for these rankings. There are a lot of publications that are considered to be more professional which are not emphasized by these databases so they get left out of the rankings.
4. Just because a journal is ranked higher than one that deals with your research, it doesn't necessarily mean you should switch. Education is a very broad category for these resources. They don't necessarily differentiate between different subcategories of education. If you're researching in Physical Education, it's not going to matter to you that a science education journal is ranked higher (unless perhaps you're working on something that crosses both areas).
Below are listed the most common ranking systems available, the provider in which you can find them and descriptions of some of the important points in this system.
Probably the most firmly entrenched systemd out there. This system doesn't have any weighting for citations. Pretty much stands as the average of citations an article gets in the journal. This systems has been in use for ages, but there are criticisms of this sytem that others below try to address.
You can access Impact Factors through Journal Citation Reports
The Article Influence score uses the Eigenfactor (below) to give an idea of the average influence an article may have in the next 5 years when published in a given journal. Higher ranked AI scores for a journal mean articles are expected to have higher influence in that journal.
You can access Article Influence Scores through Journal Citation Reports. However, a significant amount of information is also freely available from Eigenfactor.org.
The Eigenfactor score is newer and considers the source of a citation in its analysis. Citations from higher ranked journals are weighted more heavily. It also uses a broader span of time and considers disciplinary differences in citation behavior.
You can access Eigenfactors through Journal Citation Reports. However, a significant amount of information is also freely available from Eigenfactor.org.
Source Normalized Impact per Paper. This measure is weighted to account for how important getting a citation is in a given field of research (ie Physics vs Literary Criticism). It uses 4 years of data to calculate a score.
You can access SNIP Scores through Scopus
This measure weights the citations from higher ranked journals as having more weight. This is much like Google's algorithm where certain linking to yours are more important than others. A 'prestige' factor so to speak.
You can access SNIP Scores through Scopus