It used to be that the best way to discover a journal's 'Impact Factor" (ahem, the average number of citations made to the journal's content over the course of a few years...thereby implying readership) was to use the expensive, library resource ISI's Web of Science (formerly the Science Citation Index). ISI has recently taken some heat following a debunking article from the editors of Cell who were unable to replicate ISI impact factors since the company is both secretive and selective regarding the type of articles they include in their calculations.
The first, called SCImago Journal and Country Rank (SJR), uses Scopus data (a competitor to ISI),. Although not as many display options as ISI (ie. no author rankings or subject profiles) the interface is clean, intuitive, and freely open-access. Recently reviewed by Nature, SJR has caused many librarian and impact specialists to take notice and compare it to ISI Web of Science, with interesting results.
The second is Google Scholar. A simple search returns results that are based on citation counts, though the sources and coverage of journals is unclear. I recently learned a quick and dirty way of calculating the h-index by simply counting down the results of a GS "author" advanced search until the number of cites matches the rank (ie. the h paper has at least h citations each). So doing this for a UMN faculty member, I got an h-index of 37 out of 374 results (pretty close to a Web of Science count of 38).
Lets review the current impact factor tools available with an eye on their information source:
- "Journal Citation Reports" by ISI Web of Science (through library)
- SCImago Journal and Country Rank (SJR) based on Scopus data (free web based tool).
- Google Scholar ranks articles by their citation counts from undisclosed publisher data (it's google...free).
- EiganFactor.org a searchable database that displays journal rankings and results both as a list and a map (free web-based)
- Publish or Perish, software download that lists additional factors (eg. H-index) and runs off of Google Scholar (free download).