Fundamentals

Granularity

Granularity is the size of each tick: hourly, daily, weekly, SKU-store versus category-region. Finer grain looks more detailed and is usually noisier. Pick the grain the planner actually uses.

You can always add daily forecasts up to a week. You cannot honestly split a weekly forecast into days without smuggling in a shape. If ops needs days, forecast days.

SKU-store daily demand is a hard problem. Category-week is easier and still useful for buying. Many teams forecast both and reconcile. That is hierarchical forecasting, not indecision.

Changing grain changes every accuracy number. A 6% WAPE on chain-week and a 40% WAPE on SKU-day can be the same system. Quote grain with the score or the score is a toy.