A build begins with the question nobody enjoys: who is excluded? Location fields are made consistent before any comparison runs. You are given the count first and the file second.
Decay review
Identifying the records that have stopped being current, and how many of them there are.
Spelling normalisation
Settling variant forms of the same organisation name so counting does not double.
Exclusion log
A record of what was removed and why, delivered alongside the file it came out of.
Start with one segment rather than the whole plan, and see whether it holds.