xgb-subsampling-param-desc.md
col_sample_by
parameters "stack" on top of each other if several are specified. That is, given a set of 24 columns, for col_sample_by_tree=0.5
andcol_sample_by_node=0.5
,col_sample_by_tree
samples 12 columns, reducing the available, unsampled column pool to 12. col_sample_by_node
then samples half of the remaining pool, so each node samples 6 columns.
This algorithm will always sample at least one column.