We start by asking which records would be wrong to include. Gaps are measured and reported, not interpolated. Coverage is reported as three numbers: matched, unmatched, and unverifiable.
Exclusion log
A record of what was removed and why, delivered alongside the file it came out of.
Record identifiers
Giving every row a stable key so a later refresh can be matched against this one.
Supply assessment
Saying plainly where the available data stops, including when it stops short of the brief.
The first build is deliberately small, by preference.