machine-learninglisted
Install: claude install-skill anton-winter-arch/dotagents
# machine-learning
**A validation score is a claim about a construction, not a measurement of the
world.** It is only worth what the construction is worth, and most of the time
the construction is where the mistake is. A scaler fitted before the split, a
random split of dated rows, a target column that survived into the feature
matrix, a hyperparameter search that quietly consumed the test set: each of
these produces a number that is both excellent and meaningless, and none of them
looks wrong in the code.
So the order of work is fixed. Decide what is being predicted and what beats
doing nothing, construct the data so that no future or test information can
reach training, then train, and only then care which algorithm won. Algorithm
selection is the most documented and least decisive part of this, and it is the
part practitioners spend most of their time on.
Four questions answered before any model is fitted:
1. **What decision does this change?** If no decision changes, stop.
2. **What is the baseline?** Majority class, last observed value, the rule the
business already uses. Beat it or the model is not worth operating.
3. **Is the information available at prediction time?** A feature that only
exists after the event is not a feature, it is the answer.
4. **What would make this wrong in production?** Answer it now, in writing, so
the monitoring has something to watch.
## First: which situation is this?
**Framing a new problem.** Work `references/framing.md` be