As part of the analysis of the data a data mapping document should be produced  – ideally this should be signed off by the end user.

The data mapping should show the source and destinations of all fields but also should identify any fields that are not going to be migrated, these are the items that will cause most problems if there is a change in the requirements later.

In some cases there may be multiple sources for the same data item – in one migration I had three subsystems in each database to be migrated and three regional databases holding the subsystems so there were 9 sources of data for a single attribute / entity.