The other day I was curious about the historical details about the Holocaust.  I wondered how the numbers of killed were determined.  I was surprised to find that the number murdered was not determined from actual Nazi records but from European census data before and after the war.  This confuses me, because there was mass exodus of Jews and others just prior to and during the war.  Also, I read that the only way they knew who was Jewish was through self description which may have become less likely in view of the war.  What's up?