A regional holdout keeps a defined marketing change out of selected areas while applying it elsewhere. The comparison can help investigate incremental demand, but geography alone does not make an experiment credible. Different territories can have different seasonality, sales coverage and customer mix. Decide whether the regions can support a useful comparison before withholding or adding spend.
Define The Change And The Unit Of Assignment
Specify the exact campaign or budget change being evaluated. Keep ordinary customer service and unrelated acquisition activity consistent where practical. Define the geographic unit before assignment, such as a service territory whose customer records can be identified reliably. Avoid moving boundaries after results appear.
Choose the outcome and observation period in advance. If customers take time to buy, allow a suitable follow-through period and preserve their original assignment. Count outcomes using the same business definition in both conditions. A territory’s reported clicks cannot be compared directly with another territory’s completed customers.
Look For Comparable Regions And Interference
Review historical outcome levels, trends, sales staffing, offer availability and relevant local events. Matching on one month of revenue may hide different underlying patterns. Identify regions where the change could spill over through shared media, traveling customers or sellers covering both areas. Record exclusions before seeing the result.
NIST describes randomized assignment of conditions to experimental units and blocking around important nuisance factors. Those principles inform a regional design: organize comparable units where appropriate and use a planned assignment process. They do not make two arbitrarily selected cities equivalent, or remove the need for a suitable analysis and enough independent geographic units.
Distinguish A Calculation From Causal Proof
Illustrative arithmetic: treatment areas produce 100 qualified outcomes before the change and 130 afterward. Comparison areas move from 80 to 88. Treatment increases by 30 and comparison by 8. A simple difference in those changes is 22 outcomes. That number is descriptive; it is not automatically a valid estimate of incremental customers.
The interpretation depends on the design, comparable trends, scaling, spillover and other changes. A major local event in the treatment area could explain part of the increase. Different region sizes can require a different analysis. Have the analysis specified before the launch, and use qualified analytical support when the budget decision depends on the estimate.
Prepare The Operating Team For The Test
Give media and sales teams a written map of included regions, permitted changes and exceptions. Ask them to record unplanned campaign changes, staffing gaps and unusual promotions. A seller shifting effort toward treatment areas after hearing that they are important can alter the comparison.
Create a check for whether the intended treatment was actually delivered and whether the holdout remained a holdout. This is a measurement check, not permission to optimize the experiment toward a desired result. If a configuration mistake changes exposure, document it and assess whether the original question can still be answered.
Choose In Advance How The Result Will Be Used
Set guardrails for customer experience and operational capacity. Define how long the comparison will run, which events could invalidate it and what evidence would support another bounded investment. Avoid extending the test only because the current result is unfavorable or stopping it only because it briefly looks attractive.
At review, report the estimate with its uncertainty and the limitations that survived execution. If the available regions or volume cannot support a credible causal conclusion, say so. The exercise may still improve measurement or reveal geographic operating differences. It should not be used to claim proven marketing lift when the comparison cannot distinguish the campaign from those differences.
Common Questions
Is One Test Region And One Control Region Enough?
It may be useful operationally, but it can leave regional differences difficult to separate from treatment effects. Suitability depends on the design and question; do not assume a pair establishes reliable causal lift.
Can We Continue Other Campaigns?
Potentially, if they remain consistent with the analysis plan and their changes are recorded. Unplanned differences in other activity can undermine the interpretation of the tested campaign.
Sources And Further Reading
- NIST: Completely Randomized Designs · Checked 2026-10-26
- NIST: Randomized Block Designs · Checked 2026-10-26
These sources provide context. Requirements depend on the actual activity and applicable jurisdiction. Business-process guidance does not replace professional advice.