AI-900Domain 2: Describe Fundamental Principles of Machine Learning on AzureMachine Learning PrinciplesmediumSingle choice
A model achieves 99.9% accuracy on its training data but performs poorly on new production data. What is this called?
A.Underfitting
Underfitting = too simple to capture patterns — poor everywhere including training.
B.Overfitting
Memorization over generalization = overfitting. Correct.
C.Convergence
Convergence is training stabilizing — not a failure mode.
D.Data leakage by design
Leakage is a specific cause (target info in features), not the general name for this pattern.
Why this answer
Overfitting = the model memorized the training data (including its noise) instead of learning generalizable patterns — great train scores, poor real-world performance. Fixes: more data, regularization, simpler models, better splits.
Correct answer: B · Last reviewed 2026-02-14
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