I mentioned the new SCImago journal ranking site a little while back and thought I would explore it a little further. In doing so, I find that the "Cites per doc" measure, which is given for one, two, three and four year periods might be called the 'SCImago Influence Measure' or 'SIM', since it is more or less equivalent to the Web of Knowledge Impact Factor. I prefer 'influence' to 'impact', since the latter is rather macho and percussive, while the former is much more subtle and, I think, more appropriate, since what we are talking about is the influence that a journal has within its field.
The four-year SIM is particularly interesting, I think, since it allows for a much longer period of time within which the documents have a possibility of being cited. Using the SCImago database to download the data also gives the opportunity for producing some interesting comparisons. The graph below shows the four-year SIM for a long-established journal, the Journal of Documentation, compared with three, now established, open access journals - Information Research, the Journal of Digital Information and the Journal of Electronic Publishing. It is striking that on this measure all three OA journals are now approaching the same level of 'influence' as the older journal. JEP has had some problems in maintaining publication, hence the dip in 2006, but with its future now established (I believe), I imagine that the growth in its influence will resume.