Describe a correct nested split when you also search hyperparameters.
PICTURE THIS: DATA SPLIT
Fit on train, tune on val, report on test once.
Simple meaning
Keep a final test set untouched.
WHY — Train/Val/Test instead of guessing?
Why interviewers care about Train/Val/Test:
who only read docs from people who shipped.
and tied to AI / ML work.
Name the idea, why it exists, then one short example.
End with when you use it and one common pitfall.
STEPS — What happens step by step?
Before you speak the answer, walk the interviewer through these steps:
- 1Keep a final test
set untouched.
- 2On the remaining data,
run outer folds
- 3inside each outer training
piece, run inner CV or a val split to pick hyperparameters.
- 4Refit the winning setup
on the outer training data, score the outer fold, and only at the end score test once.
- 5Common mistake
What juniors usually get wrong.
- 6Close
When you pick this over the alternative.
EXAMPLE — See it in action
Here's a short line you can speak, broken into clear beats:
Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.
Key takeaway
Keep a final test set untouched. On the remaining data, run outer folds