The everyday problem
A pilot can drift when the scope expands informally or the success criteria are defined after results arrive. Both sides may believe they are evaluating different things. A short written operating plan helps keep a trial focused and makes its conclusion easier to explain.
A practical approach
Specify the workload, duration, systems, responsibilities, review process, and exit conditions. Define the baseline and measures before execution. Identify who approves scope changes and how exceptions will be counted so that inconvenient cases do not disappear from the evaluation.
Try this with your team
Write a pilot summary that fits on one page. Include a sentence describing what would cause the team to stop, revise, or expand the pilot.