Statistical reports made by the Soviet secret police agencies between the 1930s and 1950s are kept in the State Archive of the Russian Federation formerly called Central State Archive of the October Revolution (CSAOR). These documents were highly classified and inaccessible. Amid the releasing of once-secret files and democratization in the late 1980s, Viktor Zemskov and other Russian researchers managed to gain access to the documents and published the highly classified statistical data collected by the secret police and related to the number of the Gulag prisoners, special settlers, etc. In 1995, Zemskov wrote that foreign scientists have begun to be admitted to the restricted-access collection of these documents in the State Archive of the Russian Federation since 1992. However, only one historian, namely Zemskov, was admitted to these archives, and later the archives were again “closed”, according to Leonid Lopatnikov.
While considering the issue of reliability of the primary data provided by corrective labor institutions, it is necessary to take into account the following two circumstances. On the one hand, their administration was not interested to understate the number of prisoners in its reports, because it would have automatically led to a decrease in the food supply plan for camps, prisons, and corrective labor colonies. The decrement in food would have been accompanied by an increase in mortality that would have led to wrecking of the vast production program of the Gulag. On the other hand, overstatement of data of the number of prisoners also did not comply with departmental interests, because it was fraught with the same (i.e., impossible) increase in production tasks set by planning bodies. In those days, people were highly responsible for non-fulfilment of plan. It seems that a resultant of these objective departmental interests was a sufficient degree of reliability of the reports.
Between 1990 and 1992, the first precise statistical data on the Gulag based on the Gulag archives were published by Viktor Zemskov. These had been generally accepted by leading Western scholars, despite the fact that a number of inconsistencies were found in this statistics. It is also necessary to note that not all the conclusions drawn by Zemskov based on his data have been generally accepted. Thus, Sergei Maksudov alleged that although literary sources, for example the books of Lev Razgon or Aleksandr Solzhenitsyn, did not envisage the total number of the camps very well and markedly exaggerated their size, on the other hand, Viktor Zemskov, who published many documents by the NKVD and KGB, was far from understanding of the Gulag essence and the nature of socio-political processes in the country. He added that without distinguishing the degree of accuracy and reliability of certain figures, without making a critical analysis of sources, without comparing new data with already known information, Zemskov absolutizes the published materials by presenting them as the ultimate truth. As a result, Maksudov charges that Zemskov attempts to make generalized statements with reference to a particular document, as a rule, do not hold water.