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Do Not Randomly Mix Sources When Building Validation and Test Sets

If your data comes from sources with different distributions, a random split of all examples can make validation and test sets unrepresentative of the data you care about later. Those sets should match the distribution expected in deployment, not simply mirror the pooled dataset. For example, if you combine 150,000 archived desktop images with 5,000 smartphone images and then split randomly, the evaluation sets will be dominated by desktop images and will not properly reflect smartphone use.

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Updated 2026-08-12

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