You know, I have been trying to get to a good study the past few minutes. When I googled "study on the holocaust deaths" the first link is a buzzfeed article, which if you hover over the link quoted on "6million deaths" and you scroll down THAT link it states as follows:
No single wartime document
There is no single wartime document that contains the above cited estimates of Jewish deaths.
There are three obvious and interrelated reasons for the lack of a single document:
Compilation of comprehensive statistics of Jews killed by German and other Axis authorities began in 1942 and 1943. It broke down during the last year and a half of the war.
Beginning in 1943, as it became clear that they would lose the war, the Germans and their Axis partners destroyed much of the existing documentation. They also destroyed physical evidence of mass murder.
No personnel were available or inclined to count Jewish deaths until the very end of World War II and the Nazi regime. Hence, total estimates are calculated only after the end of the war and are based on demographic loss data and the documents of the perpetrators. Though fragmentary, these sources provide essential figures from which to make calculations.
