Case Study

Diagnose the evaluation-set mistake in a mobile cat-classification app.

Case context: A team trains a cat classifier using many downloaded internet images. Its dev and test sets also contain website images, but app users submit lower-resolution, blurrier, and poorly lit mobile-phone pictures. The classifier performs poorly on those uploads.

Question: What should the team diagnose, and how should it revise its dev and test sets?

Sample answer: The team should diagnose a mismatch between its website-image evaluation distribution and the mobile-image distribution it actually cares about. It should construct dev and test sets from user-uploaded mobile images because those reflect expected future inputs and the distribution on which the app should perform well. Internet images may still be used as additional training data.

Key points:

  • Website images differ from mobile uploads.
  • Current evaluation does not represent the target distribution.
  • Dev and test sets should use mobile images.
  • Internet images may remain useful for training.

Rubric: The response should identify the distribution mismatch, select mobile images for dev/test, justify that choice using future target performance, and avoid claiming that internet images must be discarded from training.

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Updated 2026-07-19

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Machine Learning

Deep Learning

Supervised Learning

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Data Science

Machine Learning Strategy

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