Underpowered Design

Suppose a valid power grid contains the following rows for a 28-period test:

effect_size	power	Monte Carlo interval	valid
0.04	0.31	0.27–0.35	true
0.06	0.55	0.50–0.60	true
0.08	0.73	0.68–0.77	true
0.10	0.84	0.80–0.88	true

At an 80% target, the grid-based MDE is 10%. This does not mean that the campaign is expected to deliver 10%, or that effects below 10% are zero. It means the configured procedure detected smaller injected effects less often than the planning threshold under this DGP.

If commercially plausible lift is 4%, the design is underpowered for that decision. Do not solve the problem by reporting an optimistic grid point or by choosing a longer period after seeing outcomes. Consider more comparable donors, lower-noise outcomes, a defensible larger treated population, a longer pre-specified measurement window, or a different design. If none is available, record the design as infeasible before launch.

Also inspect failures. A row with high estimated power and valid: false is not a usable design result. Power is divided by successful simulations, so ignored failures could otherwise make it look better than it is.