Diagnose a Weak Design

Work from failure mechanism to remedy.

Symptom	Likely issue	Action
Poor pre-period fit	Donors cannot reproduce treated dynamics	Revisit eligibility, geography level, outcome, and comparable history.
Coarse p-value granularity	Too few effective placebo assignments	Add eligible controls or reduce treated count only if the design permits it.
Dominant fitted donor weight	Counterfactual depends on one geography	Audit that donor and run exclusion sensitivity.
High power failure rate	SparseSC or DGP instability	Inspect exceptions, rank, panel completeness, and placebo feasibility.
High grid-based MDE	Noise or weak counterfactual relative to plausible lift	Redesign duration, outcome, treated scale, or method before launch.
Interference signal	Outcome changes among controls are compatible with contamination	Investigate exposure using campaign and geographic evidence; the screen is not causal proof.
Result `partial`	Missing uncertainty or warnings	Resolve the stated fields before decision use.

Do not tune exclusions, dates, or donors by searching for significance. If the remedy changes the design, rerun power and record the new specification.