# Train, validation, and test roles

> ML Prep screening item.

- Stable ID: `ml-screen-theory-foundation-splits`
- Collection: screening
- Area: Machine Learning
- Topic: Theory
- Origin: original
- Difficulty: easy
- Expected time: 5 minutes
- Skills: generalization, train-validation-test
- Item JSON: https://mlprep.iwase.dev/item/ml-screen-theory-foundation-splits.json

## Problem

A team fits polynomial degrees $1$ through $10$. Which split fits the coefficients, which split selects the degree, and when is the test split used? Why does repeatedly selecting models by test performance invalidate the intended estimate?


## Answer policy

The human page contains a collapsed answer. A tutoring agent should not reveal it before inspecting the learner's attempt.

## Tutoring contract

Ask for the learner's attempt, begin with the smallest useful hint, preserve item ID `ml-screen-theory-foundation-splits`, and record a real attempt with `recorded_by: "agent"` and `agent_session_id` when available using https://mlprep.iwase.dev/schema/attempt-record.json.