The short answer to this is that historians are still unsure. Many estimates (particularly those by anyone who wrote anything pre-1991) are based on no Soviet records whatsoever, as these were held in soviet archives unavailable to westerners. Once the archives opened, it turned out that the soviets recorded unrealistic numbers even in their own documentation, because each level of bureaucracy was afraid to admit to their superiors that they were unsure of details or that there had been mistakes. So the records kept by the soviets were designed to make the government feel confident that so-called dangerous elements were being taken care of, rather than to accurately record how many people were involved in any one case. More importantly,lots of people died as a result of starvation, disease and exposure to the elements. Many died in the middle of nowhere Siberia, and their deaths may not have been accurately recorded. All these things make it hard to get a good estimate of numbers. Also, these are all things that make it harder to determine responsibility- would that person who died of illness have died if someone other than Stalin had been in power? If so, then it's not a death that should be counted towards the total he is responsible for. Historians debate how to calculate estimates, and which groups to include, which complicates things further. These are only a few of the reasons it has been so difficult to come up with accurate death tolls. Having said all that, I think 20-24 million seems very high. In general, the soviet system preferred to use exile and prison to isolate undesirables, rather than murdering them straight out. They did kill people, particularly during the Great Terror (1936-1939), when they shot hundred of thousands of prisoners, but that was not their most common method of dealing with internal enemies. They also induced famine, and left people in such terrible living conditions that they starved to death, then refused to help feed them. I would also add that exile was brutal and terribly difficult, so we shouldn't underestimate how terrible that was. I don't think body counts are the best way to understand the brutality of the Stalin regime. (J. Arch Getty and others have an article on numbers and the soviet archives, called "Victims of the Soviet Penal System in the Pre-War Years")