2011/3/11 Amir E. Aharoni amir.aharoni@mail.huji.ac.il:
The English version says: "See the study for more detailed breakdowns of cohorts of individual editors over time, and for data from other languages."
See http://en.wikipedia.org/wiki/Cohort_(statistics)
It's a term that refers to a group that shares certain characteristics, typically a time period. In this case it refers to a group of editors who started editing in a given month, e.g. January 2008, and it follows the development of that group over time.
If you can't find a better translation, the equivalent of "group" would be fine.