In gbm funtion, if we set train.fraction=0.5, cv.fold=5. Does gbm fit twice for trained sample and CV sample,respectively? 1) 50% of sample is used as trained data to fit the 1st model. 2) 80% of sample is used as CV sample to fit the second model
If that is the case, why gbm.perf only reports the same error curve for both the cases? The only thing differs is just the best iteration number....
If not the case, how does gbm work behind the scene?
Thank you very much!