Soviet repressions of Polish citizens occurred as well over this period. Since Poland's conscription system required every nonexempt university graduate to become a military reserve officer,[15] the NKVD was able to round up a significant portion of the Polish educated class.[f] According to estimates by the Institute of National Remembrance (IPN), roughly 320,000 Polish citizens were deported to the Soviet Union (this figure is questioned by some other historians, who hold to older estimates of about 700,000-1,000,000).[16][17] IPN estimates the number of Polish citizens who died under Soviet rule during World War II at 150,000 (a revision of older estimates of up to 500,000).