title: subsample, colsample_bytree, colsample_bylevel in XGBClassifier() Python 3.x
body: <p>I've spent a good deal of time trying to find out what these "subsample", "colsample_by_tree", and "colsample_bylevel" actually did in XGBClassifier() but I can't exactly find out what they do. Can someone please explain briefly what it is they do?</p>
  
  <p>Thanks!</p>