Question: In a concise analytical response, explain why training performance by itself or held-out performance by itself does not fully capture the informal meaning of variance.

Sample answer: Informal variance refers to how much worse a model performs on a validation or test set than it does on the training set. That means the concept is based on a comparison between two performance levels. If you look at only one result, you do not see the performance gap that the term is meant to describe.

Key points:

• Training performance provides the baseline.
• Validation or test performance is compared against that baseline.
• Variance is the amount of degradation from training to held-out data.
• One performance number alone does not show the gap.

Rubric: A strong response identifies both parts of the comparison, explains that variance is the difference between them, and states why either result alone is incomplete.