Protect the Control Group From Contamination
Identify common ways control and treatment drift into each other and design safeguards before launch.
The baseline only helps if it stays a baseline. Assignment discipline Random assignment creates comparable groups, but manual exceptions after launch can undo that benefit quickly. Decide exception rules before pressure arrives. Variant-specific experience Emails, support scripts, holdout logic, and logging should match the assigned experience. Shared downstream actions can contaminate the comparison even when the interface differs. Signal dilution When control users receive treatment elements, the measured effect shrinks and the test may look powerless or misleadingly flat even if the idea truly helps.
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