when `mtry =1` all predictors are used in xgboost engine

[joeycouse]

Whenever mtry = 1 in the model spec the corresponding value of colsample_bytree is not converted to proportion correctly.
example: when mtry = 1 , colsample_bytree = 1 when it should be 0.125.

library(tidyverse)
library(tidymodels)
#> Registered S3 method overwritten by 'tune':
#>   method                   from   
#>   required_pkgs.model_spec parsnip
library(mlbench)
library(xgboost)
#> 
#> Attaching package: 'xgboost'
#> The following object is masked from 'package:dplyr':
#> 
#>     slice
library(reprex)


data("PimaIndiansDiabetes")
df <- PimaIndiansDiabetes %>%
  mutate(diabetes = fct_relevel(diabetes, 'pos'))

#Model with mtry = 1, col_sample_by_tree should be 1/8 = 0.125

tidy_model <- 
  boost_tree(trees = 10,
             mtry = 1,
             tree_depth = 3) %>%
  set_engine('xgboost', 
             eval_metric = 'auc',
             event_level = "first",
             verbose = 1) %>%
  set_mode('classification')

set.seed(24)

tidy_model_fitted <- tidy_model %>% 
  fit(diabetes ~ . , data = df)
#> [1]  training-auc:0.834787 
#> [2]  training-auc:0.853881 
#> [3]  training-auc:0.873048 
#> [4]  training-auc:0.875000 
#> [5]  training-auc:0.882873 
#> [6]  training-auc:0.891716 
#> [7]  training-auc:0.896067 
#> [8]  training-auc:0.900414 
#> [9]  training-auc:0.904313 
#> [10] training-auc:0.906937

#Model with mtry  = 2, col_sample_by_tree should by 2/8 = 0.25
tidy_model_2 <- 
  boost_tree(trees = 10,
                         mtry = 2,
                         tree_depth = 3) %>%
  set_engine('xgboost', 
             eval_metric = 'auc',
             event_level = "first",
             verbose = 1) %>%
  set_mode('classification')

set.seed(24)
tidy_model_fitted_2 <- tidy_model_2 %>% 
  fit(diabetes ~ . , data = df)
#> [1]  training-auc:0.684049 
#> [2]  training-auc:0.776993 
#> [3]  training-auc:0.802187 
#> [4]  training-auc:0.808041 
#> [5]  training-auc:0.830478 
#> [6]  training-auc:0.837851 
#> [7]  training-auc:0.847817 
#> [8]  training-auc:0.847254 
#> [9]  training-auc:0.850034 
#> [10] training-auc:0.856455

#col_sample by tree should be 1/8 = 0.125, instead model is using all predictors
tidy_model_fitted$fit$call$params$colsample_bytree
#> [1] 1

#colsample_bytree is correct 2/8 = 0.25
tidy_model_fitted_2$fit$call$params$colsample_bytree
#> [1] 0.25

^{Created on 2021-04-06 by the reprex package (v1.0.0)}

[juliasilge]

I think the problem is here:

parsnip/R/boost_tree.R

Lines 366 to 371 in 30dc026

	if (colsample_bytree > 1) {
	colsample_bytree <- colsample_bytree/p
	}
	if (colsample_bytree > 1) {
	colsample_bytree <- 1
	}

It doesn't convert the colsample_bytree == 1 case, assuming it is meant as "use all the predictors".

• How would colsample_bytree get to this function as a proportion and not a count? (I can't find a way.)
• If it is possible, is there a way to disambiguate this?

[juliasilge]

This problem will now apply to both colsample_bytree and colsample_bynode.

[juliasilge]

Results now:

library(tidymodels)
#> Registered S3 method overwritten by 'tune':
#>   method                   from   
#>   required_pkgs.model_spec parsnip
library(mlbench)

data("PimaIndiansDiabetes")
df <- PimaIndiansDiabetes %>%
  mutate(diabetes = forcats::fct_relevel(diabetes, 'pos'))


tidy_model <- 
  boost_tree(trees = 10,
             mtry = 1,
             tree_depth = 3) %>%
  set_engine('xgboost', 
             eval_metric = 'auc',
             event_level = "first",
             verbose = 1) %>%
  set_mode('classification')

set.seed(24)

tidy_model_fitted <- tidy_model %>% 
  fit(diabetes ~ . , data = df)
#> [1]  training-auc:0.731097 
#> [2]  training-auc:0.821153 
#> [3]  training-auc:0.836787 
#> [4]  training-auc:0.854131 
#> [5]  training-auc:0.866493 
#> [6]  training-auc:0.876743 
#> [7]  training-auc:0.878963 
#> [8]  training-auc:0.879940 
#> [9]  training-auc:0.884306 
#> [10] training-auc:0.887933

# col_sample by node should be 1/8 = 0.125 
tidy_model_fitted$fit$call$params$colsample_bynode
#> [1] 0.125

^{Created on 2021-05-21 by the reprex package (v2.0.0)}

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