Minimal-Prior Policy

Purpose

Use a principled but low-friction prior setup that avoids specification-hunting while preserving identifiability in short, collinear MMM datasets.

Policy

1. Default-first: keep priors.use_defaults: true.
2. Sparse overrides: only add priors.overrides for high-conviction terms.
3. Selective bounds: add boundaries.overrides only for structural signs.
4. No blanket constraints: do not force all controls/media to one sign by default.
5. Diagnose before tightening: use pre-flight and diagnostics gates first, then add priors/bounds if uncertainty is still unstable.

YAML mapping

priors:
  use_defaults: true
  overrides:
    # Optional high-conviction override examples:
    # - { parameter: price_index, mean: -0.2, sd: 0.1 }
    # - { parameter: distribution, mean: 0.15, sd: 0.1 }

boundaries:
  overrides:
    # Optional structural sign constraints:
    # - { parameter: m_tv, lower: 0.0, upper: .Inf }
    # - { parameter: competitor_discount, lower: -Inf, upper: 0.0 }

When to override defaults

• Do override when domain mechanism is stable and defensible.
• Do not override only to improve one run’s fit metrics.
• Do not add bounds if sign can plausibly flip under promotion, pricing, or substitution effects.

Review checklist

• Are overrides fewer than the number of major business assumptions?
• Is each bound tied to a concrete causal rationale?
• Did diagnostics indicate a real identifiability problem before tightening?