Similar accounts, same tier, segment and access state, that did not receive the effort in the same period. Their change carries the season, the market and the product change; subtracting it leaves what is associated with the effort. It removes noise, not the selection of who got the effort.
Own change alone credits the market to the effort.
A holdout is random and measures cause. A comparison group is observational and measures association.
Accounts of the same tier, segment and status that did not receive the effort in the same period. The outcome for the treated accounts is read against the outcome for this group, not against their own past alone.
Accounts called monthly grew 6 percent. Similar accounts not called grew 1 percent over the same months. The difference of 5 points is the most that can be attributed to the calls.
Before and after on the treated accounts only. The market rose 4 percent, and the programme is credited with all 6.