DIAG-8 · Diagnosis · Invariant — holds by construction, no expiry
Window integrity
The law
Time-additive ad metrics are defined only over a stated window: a valid sum requires rows that exactly partition that window, no gaps, overlaps, or mixed coverage. Partial coverage may be reported as-is or as a labeled estimate, never as the period total.
Scope
Flow metrics (spend, impressions, clicks, conversions and their ratios) on one stated time axis. Stock metrics (Quality Score, budget) are point-in-time reads, latest-row-per-entity is correct there and only there.
What this licenses you to do
For every per-period figure, verify the summed rows exactly tile the labeled window on one time axis; report shortfalls as "over N days," never as the period.
Evidence
Observed in production: a wasted-spend calculation that silently used the wrong date window overstated the true figure by a wide margin until the window arithmetic was corrected, exactly the kind of error this law predicts when window integrity isn't checked.