Case Study

Diagnosing a model with 14% training error, 15% train-dev error, and 16% dev error

Case context: A team is building a fraud detection model for online payments. Its measured error rates are 14% on the training set, 15% on the training-dev set, and 16% on the dev set.

Question: What is the most likely problem indicated by these numbers, and what do they suggest about the model's performance on the training data?

Sample answer: The most likely issue is high avoidable bias. The model is already making many mistakes on the training set, so it is not fitting the training data well and is underperforming there.

Key points:

  • Identify high avoidable bias as the main problem.
  • Recognize that training error is already substantial.
  • Conclude that the model is not learning the training examples effectively.

Rubric: The response must identify the problem as high avoidable bias and clearly state that the model is performing poorly on the training data.

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Updated 2026-08-12

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