Twitter added support for "@replies" beginning in May 2008,[3] with any combination of @ with a username being turned into a hyperlink to the profile. On March 30, 2009, Twitter updated the feature and renamed it "Mentions" (i.e., to "mention" user "@janedoe") so as to include non-reply posts directed at individual users.[4]
With pk=n_k/n being the fraction of the n_k samples from class k = {0,1} out of the total of n samples at node τ, the Gini impurity i(τ) is calculated as 1-p_0^2-p_1^2. The more discriminativ or clean a node is the lower is the value. A split will happen, if the decrease (delta_i) of the value is great for a given node. The gini decrease is the sum over all nodes in all trees of their delta_i's.