The recent article in Slate regarding the AskHistorians policy regarding Holocaust denial was interesting. One of the central thrusts seemed to be that outright denial of the Holocaust is certainly bad, but the more insidious form of 'just asking questions' with the intent to ferment doubt and uncertainty regarding the facts is equally bad or worse. So it is with some care that I ask this, and I offer (for whatever it may be worth) an assurance that my intentions are pure. I am in no way questioning what happened, or the numbers in any significant sense, but just trying to understand where a tranche of the data comes from.

I understand that much of the data regarding the number of people murdered during the Holocaust comes from the Reich's own (in)famously meticulous records from the extermination camps and concentration camps, and that these have been corroborated with pre-war residential and population data from Europe, as well as from surviving family accounts and records. This would seem a complex task, but at least a relatively straightforward one. But large numbers of people were killed, especially earlier in the war, by a combination of mobile SS groups (Einsatszgruppen) who murdered people in groups in an ad-hoc manner as the Reich forces advanced in what would be assumed to be a far less well-documented fashion. Equally, the regime employed native nationalist/xenophobic paramilitiaries to enact pogroms and mass murders in their stead in many areas across eastern Europe in the same period. I would assume that these groups did not keep particularly good records themselves, and in some cases these actions were occurring with a backdrop of a similarly extraordinary destruction of the state and population (I understand that Belarus lost a quarter of its total population and almost four-fifths of its settlements destroyed). It would nevertheless appear that historians have come to reasonably precise estimates of the numbers killed.

My question is therefore: how do historians arrive at the numbers murdered by the SS and their analogues in the east outside of the extermination camp system?