The researchers collected data on how many papers were cited a certain numbers of times in a range of scientific fields, and found that the raw numbers varied widely from one discipline to another. For example, for papers published in 1999, articles with 100 citations are 50 times more common in developmental biology than in aerospace engineering.
But if the citation counts are divided by the average number of citations per paper for the discipline in that year, the resulting statistical distributions are remarkably similar. All fit very precisely on a single curve, corresponding to what statisticians call a 'log-normal' distribution. This relationship is "amazingly clean", says Fortunato. The method allows direct comparison between disciplines — for example, an article published in aerospace engineering in 1999 that gained 20 citations had more impact in its field than an article with 100 citations in developmental biology.
I suppose with this, the citation index, and the h-index, we have a lot of tools at our disposal to evaluate the impact of not just an individual, but also whole fields and whole departments.