---
name: spillmark-holdout-test
description: Size a geo holdout test from a seller's own Amazon sales before a brand is connected. Use when asked how long to pause ads, how small a lift a test can see, or whether a Meta-to-Amazon test is big enough.
---

# Holdout test size

This skill sizes a test. It does not read a connected Amazon account, lock a plan, or report a measured result.

1. If the Spillmark MCP server is connected, call `run_calculator` with slug `holdout-test-calculator`.
2. Use the seller's own numbers. Do not send the calculator's example values as if they were the brand's.
3. Enter percents as points: 15 means 15%.
4. Inputs are `baseline` (weekly Amazon sales in one region), `cv` (week-to-week variability), `regions` (usually US states), `lift` (the lift to plan for), `weeks`, `confidence`, `power`, and optional `spend` (weekly Meta spend).
5. Repeat the tool's result and its interpretation, including the 95% wording it returns. If it says the test is too small, say that.
6. Give the seller the `url` from the tool so they can open the same inputs.
7. Stop there. A locked plan and a scored result need a connected brand. Do not invent either.

Server: https://spillmark.com/mcp

Calculator: https://spillmark.com/tools/holdout-test-calculator