Probabilities returned by multi:softprob

Can someone help me understand how the multi-class probabilities are calculated?

Is it calculated during bagging by simply counting how many ensembles predicted the label, divided by total ensembles?

No, we use one-vs-rest method to classify multi-class data. So if you run K boosting rounds, you will obtain K * C trees, where C is the number of classes. At prediction time, we group K * C trees into C groups and compute partial sums for each group, obtaining C scores. Finally, we take the softmax to convert the C scores into probabilities.

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Thank you for your reply!