Thank them, restate the paper’s claim, explain why the extra experiment would answer a different question, and offer a smaller feasible check if one exists. Put the out-of-scope work in Future Work.
Example Question Reviewer Response
Computer Science
Publication Stage
How do I respond when Reviewer 2 asks for experiments outside scope?
Example of the kind of research problem you can ask on PhDSOS.
They want a new dataset and a method I did not claim. How do I decline without sounding defensive?
Answers
Help this researcher move forward
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