Hi,

Can anyone explain why a sudden change in impact factor for a journal should occur between one year and the next?

The journal I am thinking about is Molecular Biology and Evolution, IF 2008: 7.28 -> IF 2009: 9.87 (!)

Can anyone verify this? Did they change the way they calculate it or something?

Why not? If the journal published a lot of good/popular/highly cited articles, then the numbers can change quite a bit from one year to another.
I think there is the IF list for 2009 somewhere on this site. For 2008, I also have 7.28

It also shows that the impact factor doesn't mean too much -- publish good papers, and they'll get cited. It could also go the other way -- as they're fond of pointing out in financial circles, past performance is no guarantee of future results.

I am also getting the feeling that most IFs increased during the last years, but I suspect this is because more and more journals are included in the ranking .... the more journals - the more citations - the higher the IF