Essay

Compare the diagnostics of poor test performance under same vs. different dev/test distributions.

Question: Suppose a model performs well on the dev set but poorly on the test set. Analyze how the diagnosis of this issue and the options for fixing it differ when the dev and test sets come from the same distribution versus when they come from different distributions.

Sample answer: If the dev and test sets come from the same distribution, the diagnosis is clear: the system has overfit the dev set, and the obvious cure is to obtain more dev set data. If the dev and test sets come from different distributions, the diagnosis is ambiguous and options are less clear. The failure could be because the system overfit the dev set, the test set is harder than the dev set, or the algorithm is doing as well as could be expected.

Key points:

  • Under the same distribution, poor test set performance clearly indicates the model has overfit the dev set.
  • The obvious cure for dev set overfitting when distributions match is to collect more dev set data.
  • If distributions differ, diagnosing poor test performance becomes ambiguous and options are less clear.
  • Potential reasons for failure under different distributions include dev set overfitting, a harder test set, or the algorithm doing as well as expected.

Rubric: To receive full credit, the answer must identify that: 1. Under the same distribution, the diagnosis is overfitting the dev set and the cure is getting more dev set data. 2. Under different distributions, the diagnosis is ambiguous. 3. The potential causes under different distributions include: overfitting the dev set, the test set being harder, or the algorithm doing as well as could be expected.

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Updated 2026-05-26

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