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I am trying to understand how an optimal tree length is determined using cost-complexity pruning.

Visit this lecture note screenshot: https://www.dropbox.com/s/qgr742zg45ahqgr/Screenshot%202015-02-04%2018.17.03.png?dl=0

In the highlighted equation, I don't understand how C(T) or alpha is calculated. My thought process is that a large tree is grown and then a subtree is created and then they are compared but I don't understand how.

Thanks in advance.

Blou91
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  • What don't you understand about how to calculate C(T)? Given a value for \alpha, you highlighted the equation. – goodepic Feb 05 '15 at 05:49
  • I don't understand then how values of alpha are calculated or selected? – Blou91 Feb 06 '15 at 17:18
  • Take a look at this [scikit-learn tutorial](https://scikit-learn.org/stable/auto_examples/tree/plot_cost_complexity_pruning.html). It might enlighten you. – Tanguy Apr 07 '20 at 11:01

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