By the time of Ssu-ma Kuang, in the Northern Sung, there was a great deal of manpower available, and there was much firsthand material, so that collection of data was extremely easy, and few obstacles were encountered.
而到北宋時的司馬光，則有眾多人力可供驅策，現成史料也很多，採擷甚易，遭遇的困難較少。