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Case Study

Interpreting Training Error on a Large Dataset

Case context: A developer fits a neural network on 80,000 labeled records and finds that it still misclassifies 12% of the training examples.

Question: In the informal sense used in the course material, what does this 12% training error indicate, and why does the size of the training set matter for that interpretation?

Sample answer: The 12% training error is a practical estimate of the model’s bias. In this informal usage, bias means the model’s error on the training data. Because the training set is very large, that error rate is a reasonable stand-in for bias rather than a number that is heavily distorted by a tiny sample.

Key points:

  • The 12% error on the training set is taken as an estimate of bias.
  • A very large training set makes the training error a useful approximation of bias.

Rubric: The response must identify the 12% training error rate as bias under the informal definition, and note that a very large training set is what makes the training error a good proxy for bias.

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Updated 2026-08-12

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