 He suggests that the Tribal Hidage may have been a tribute list drawn up by Edwin of Northumbria in the 620s, and that it probably originated before 685, after which no Northumbrian king exercised "imperium" over the Southumbrian kingdoms.According to Higham, the values assigned to each people are likely to be specific to the events of 625-626, representing the individual contracts made between Edwin and those who recognised his overlordship at that time.This explains the artificial and rounded nature of the figures that were arrived at