# Who wrote the formula for gini importance/sklearn's feature importance score?

I've been looking for a paper where the Gini importance was first proposed, but I am not sure if this is actually how it came to be.

Here's the formula I am familiar with and am looking to find in a paper:

$$\frac{N_s}{N_t} * \left(i - \frac{{N_s}_r}{N_s} i_r - \frac{{N_s}_l}{N_s} i_l \right)$$

where

$$N_s$$ = number of samples at a particular node

$$N_t$$ = number of total samples

$$i$$ = Impurity

$$*_r$$ = measure of the right child node

$$*_l$$ = measure of the left child node

If I understand properly, this is the formula sklearn's random forests also use in model.feature_importances_

Tags (since I don't have the rep to create new ones yet): Feature Importance Score, Gini Importance