KupriyanovB., KosaretskyS., MertsalovaT., and SemenovaT. “Supplementary Education Institutions for Children (Data from Center for Statistics and Monitoring of Education)”. Voprosy Obrazovaniya / Educational Studies Moscow, no. 2, Oct. 2013, pp. 209-31, doi:10.17323/1814-9545-2013-2-209-231.