Promotion lift is the difference between actual sales and a credible no-promotion baseline. Counting all campaign-period sales as lift includes units that would have sold anyway.
The demo overlays actual sales and a baseline, highlighting only the extra portion. Change the baseline to see how the estimated lift changes even with the same observed sales.
Account for seasonality, advertising, pulled-forward purchases and post-event dips. Subtract discount cost and cannibalization to assess profit.
When to use
Define a baseline before comparing campaigns or allocating the next budget.