Relation

Chain of Assumptions in Supervised Statistical Learning

A supervised learning system is expected to perform well only when a chain of four assumptions holds, each verified on a different dataset:

  1. Low bias: the model fits the training set well on the cost function (e.g., comparable to human-level performance).
  2. Low variance: the model fits the dev (validation) set well on the cost function.
  3. The model fits the test set well on the cost function.
  4. The model performs well in the real world (e.g., happy users).

Because the checks are sequential, a failure at a later step points to the stage that needs attention (e.g., high dev-set error indicates a variance problem).

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Updated 2026-07-20

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Data Science