Gini Splits

What One Split Is and Is Not

The finale separates exact split arithmetic from broader tree-building choices. It states the boundary of what the rendered counts justify.

highlighted = computed this step

What is exact

Counts, Gini impurity, weighted impurity, and gain are exact for the shown rows. Feature A gain is 1/18.

gainA=1/18\text{gain}_A=1/18
Exact split mechanicsThe final view keeps the chosen split visible.Chosen Feature A splitroot splitcounts=[3, 3]G=1/2left leafcounts=[2, 1]G=4/9right leafcounts=[1, 2]G=4/9

What this surface stops at

This surface renders one split only. Deeper tree choices would need their own displayed counts and checks.

one split shown\text{one split shown}
Exact split mechanicsThe final view keeps the chosen split visible.Chosen Feature A splitroot splitcounts=[3, 3]G=1/2left leafcounts=[2, 1]G=4/9right leafcounts=[1, 2]G=4/9

Summary

This pins the split mechanics on toy counts; it is no claim about depth, pruning, overfitting, accuracy, or generalization.

toy-count split mechanics only\text{toy-count split mechanics only}
Exact split mechanicsThe final view keeps the chosen split visible.Chosen Feature A splitroot splitcounts=[3, 3]G=1/2left leafcounts=[2, 1]G=4/9right leafcounts=[1, 2]G=4/9