sentences:  Hata's major contribution to Nanking Massacre studies is his book "Nankin jiken" ("The Nanking Incident"), published in 1986, which is a detailed study of the event based on Japanese, Chinese, and English sources that was later noted by historians such as Daqing Yang to be one of the few impartial works of scholarship written on the massacre during the period.The book is known for its relatively low estimate of the death toll, which Hata put at up to 40,000 because he based the number of civilian killing on the work of Lewis S. C. Smythe who conducted survey of massacre in the immediate aftermass (War Damage in Nanking Area, Dec.1937 to March 1938, Urban and Rural Surveys) and also exclude Chinese soldiers.Hata's book is acknowledged as the first to discuss what might have caused the massacre, whereas previous books had focused only on the event itself.