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Choose Dev and Test Sets to Match the Main Goal When Feasible

After the development and test sets are defined, model selection will usually be driven by dev-set results, so the dev set should represent the real task the team most wants to optimize. If dev and test data come from different sources, a model may look strong on dev data yet fail on test data, and the reason for that gap may be unclear. When both sets are drawn from the same distribution, that same gap has a much cleaner interpretation: the model is fitting the dev set too closely, and the next step is often to gather more dev-set examples. If the two sets are mismatched, the test gap could also mean the test task is harder or simply different, which makes diagnosis much less certain.

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Updated 2026-08-12

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