The MMM glossary
Every term below will appear in a vendor deck or an AE call. Each entry opens with a snippet-length definition, then a worked example with real arithmetic. Terms link laterally, wiki-style; start anywhere.
Why an ad's effect outlives the week it ran, and the decay rate that describes how fast it fades.
SaturationThe bending curve that makes the second million buy less than the first; the Hill function in plain English.
CarryoverThe lag between spend and effect, how it differs from adstock, and what mis-specifying it does to ROI.
Base vs incrementalThe core decomposition: revenue that would have arrived anyway versus revenue marketing caused.
IncrementalityThe quantity MMM estimates and dashboards do not: sales that vanish if the spend does.
Halo effectCross-channel spillover, TV lifting branded search, and what ignoring it does to budgets.
CannibalizationWhen paid channels eat sales that organic channels would have made, inflating apparent ROAS.
Diminishing returnsWhy extrapolating last month's ROAS to double the budget fails, every time.
ROI vs ROASTwo formulas, routinely confused, and the marginal version that should drive allocation.
Bayesian priorsWhat a prior is, why vendors argue about them, and how they can be abused.
Pre-reading for a vendor call: start with incrementality, priors, and saturation, then take the 12 questions with you. Deeper treatments live on the learn shelf.