The Tsar and the Russian Empire used both forced exile and forced labor as forms of judicial punishment. Katorga, a category of punishment reserved for those convicted of the most serious crimes, had many of the features associated with labor-camp imprisonment: confinement, simplified facilities (as opposed to prisons), and forced labor, usually involving hard, unskilled or semi-skilled work. According to historian Anne Applebaum, katorga was not a common sentence; approximately 6,000 katorga convicts were serving sentences in 1906 and 28,600 in 1916. Under the Imperial Russian penal system, those convicted of less serious crimes were sent to corrective prisons and also made to work. Forced exile to Siberia had been in use since the seventeenth century for a wide range of offenses and was a common punishment for political dissidents and revolutionaries. In the nineteenth century, the members of the failed Decembrist revolt, Polish nobles who resisted Russian rule, and members of various socialist revolutionary groups, including Bolsheviks such as Sergo Ordzhonikidze, Leon Trotsky, and Joseph Stalin were all sent into exile. Convicts serving labor sentences and exiles were sent to the underpopulated areas of Siberia and the Russian Far East – regions that had few towns or food sources and lacked any organized transportation systems. Despite the isolated conditions, there were prisoners who successfully escaped to populated areas. Stalin himself escaped three of the four times he was sent into exile. From these times, Siberia gained its fearful connotation of punishment, which was further enhanced by the Soviet GULAG system. The Bolsheviks' own experiences with exile and forced labor provided them with a model on which to base their system, including the importance of strict enforcement.