Classification example - HR data

Anna Gierlak

2021-03-29

In this vignette we demonstrate an application of the rSAFE package to the HR_data set. The dataset contains information from a Human Resources department about employees and may be used in the classification task of predicting whether an employee is likely to leave the company. The data comes from the Kaggle competition “Human Resources Analytics” and is available in breakDown and rSAFE packages.

library(rSAFE)
head(HR_data)
#>   satisfaction_level last_evaluation number_project average_monthly_hours
#> 1               0.38            0.53              2                   157
#> 2               0.80            0.86              5                   262
#> 3               0.11            0.88              7                   272
#> 4               0.72            0.87              5                   223
#> 5               0.37            0.52              2                   159
#> 6               0.41            0.50              2                   153
#>   time_spend_company work_accident left promotion_last_5years sales salary
#> 1                  3             0    1                     0 sales    low
#> 2                  6             0    1                     0 sales medium
#> 3                  4             0    1                     0 sales medium
#> 4                  5             0    1                     0 sales    low
#> 5                  3             0    1                     0 sales    low
#> 6                  3             0    1                     0 sales    low

As explanatory variables we use all available in the dataset except for which specifies department in which the employee works for.

data <- HR_data[, colnames(HR_data) != "sales"]

In order to ensure the final errors are computed on the data which has not been seen by an appropriate model, we divide our data as follows:

set.seed(111)
data <- data[sample(1:nrow(data)),]

data1 <- data[1:4000,]
data2 <- data[4001:8000,]
data3 <- data[8001:12000,]
data4 <- data[12001:14999,]

Building a black-box model

In this example we decide to use the GBM model as a black-box - it will serve us as a surrogate.

n.trees <- 10000
library(gbm)
set.seed(111)
model_xgb1 <- gbm(left ~ ., data = data1, distribution = "bernoulli", n.trees = n.trees)

Creating an explainer

We also create an explainer object that will be used later to create new variables. For classification problems we need to specify predict_function - a function that may be used for model predictions and returns a single numerical value for each observation.

library(DALEX)
predict_function <- function(model, x) predict(model, x, type = "response", n.trees = n.trees)
explainer_xgb1 <- explain(model_xgb1, data = data2[,-7], y = data2$left, predict_function = predict_function, verbose = FALSE)

Creating a safe_extractor

Now, we create a safe_extractor object using rSAFE package and our surrogate model. Setting the argument verbose=FALSE stops progress bar from printing.

safe_extractor <- safe_extraction(explainer_xgb1, penalty = 20, verbose = FALSE)

Now, let’s print summary for the new object we have just created.

print(safe_extractor)
#> Variable 'satisfaction_level' - selected intervals:
#>  (-Inf, 0.48]
#>      (0.48, 0.9193878]
#>      (0.9193878, Inf)
#> Variable 'last_evaluation' - selected intervals:
#>  (-Inf, 0.42]
#>      (0.42, 0.57]
#>      (0.57, 0.8]
#>      (0.8, Inf)
#> Variable 'number_project' - selected intervals:
#>  (-Inf, 4]
#>      (4, Inf)
#> Variable 'average_monthly_hours' - selected intervals:
#>  (-Inf, 115.6122]
#>      (115.6122, 160]
#>      (160, 214]
#>      (214, Inf)
#> Variable 'time_spend_company' - selected intervals:
#>  (-Inf, 3]
#>      (3, Inf)
#> Variable 'work_accident' - no transformation suggested.
#> Variable 'promotion_last_5years' - no transformation suggested.
#> Variable 'salary' - created levels:
#>  high ->  high 
#>  low, medium ->  low_medium

We can see transformation propositions for all variables in our dataset.

In the plot below we can see which points have been chosen to be the breakpoints for a particular variable:

plot(safe_extractor, variable = "last_evaluation")

For factor variables we can observe in which order levels have been merged and what is the optimal clustering:

plot(safe_extractor, variable = "salary")

Transforming data

Now we can use our safe_extractor object to create new categorical features in the given dataset.

data3_trans <- safely_transform_data(safe_extractor, data3, verbose = FALSE)
salary satisfaction_level last_evaluation number_project average_monthly_hours time_spend_company work_accident left promotion_last_5years satisfaction_level_new last_evaluation_new number_project_new average_monthly_hours_new time_spend_company_new salary_new
low 0.78 0.77 2 177 4 0 0 0 (0.48, 0.9193878] (0.57, 0.8] (-Inf, 4] (160, 214] (3, Inf) low_medium
low 0.87 0.94 5 219 5 0 1 0 (0.48, 0.9193878] (0.8, Inf) (4, Inf) (214, Inf) (3, Inf) low_medium
medium 0.85 1.00 5 244 2 0 0 0 (0.48, 0.9193878] (0.8, Inf) (4, Inf) (214, Inf) (-Inf, 3] low_medium
medium 0.24 0.46 7 224 5 0 1 0 (-Inf, 0.48] (0.42, 0.57] (4, Inf) (214, Inf) (3, Inf) low_medium
low 0.81 0.49 4 285 4 0 0 0 (0.48, 0.9193878] (0.42, 0.57] (-Inf, 4] (214, Inf) (3, Inf) low_medium
high 0.72 0.84 2 173 2 1 0 0 (0.48, 0.9193878] (0.8, Inf) (-Inf, 4] (160, 214] (-Inf, 3] high

We can also perform feature selection if we wish. For each original feature it keeps exactly one of their forms - original one or transformed one.

selected_variables <- safely_select_variables(safe_extractor, data3_trans, which_y = "left", verbose = FALSE)
data3_trans_sel <- data3_trans[,c("left", selected_variables)]
print(selected_variables)
#> [1] "work_accident"             "promotion_last_5years"    
#> [3] "salary"                    "satisfaction_level_new"   
#> [5] "last_evaluation_new"       "number_project_new"       
#> [7] "average_monthly_hours_new" "time_spend_company_new"

It can be observed that for some features the original form was preferred and for others the transformed one.

Here are the first few rows for our data after feature selection:

left work_accident promotion_last_5years salary satisfaction_level_new last_evaluation_new number_project_new average_monthly_hours_new time_spend_company_new
0 0 0 low (0.48, 0.9193878] (0.57, 0.8] (-Inf, 4] (160, 214] (3, Inf)
1 0 0 low (0.48, 0.9193878] (0.8, Inf) (4, Inf) (214, Inf) (3, Inf)
0 0 0 medium (0.48, 0.9193878] (0.8, Inf) (4, Inf) (214, Inf) (-Inf, 3]
1 0 0 medium (-Inf, 0.48] (0.42, 0.57] (4, Inf) (214, Inf) (3, Inf)
0 0 0 low (0.48, 0.9193878] (0.42, 0.57] (-Inf, 4] (214, Inf) (3, Inf)
0 1 0 high (0.48, 0.9193878] (0.8, Inf) (-Inf, 4] (160, 214] (-Inf, 3]

Now, we perform transformations on another data that will be used later to compare models performance.

data4_trans <- safely_transform_data(safe_extractor, data4, verbose = FALSE)
data4_trans_sel <- data4_trans[,c("left", selected_variables)]

Creating white-box models on original and transformed datasets

Let’s fit the models to data containing newly created columns. We consider a generalized linear model (glm) as a white-box model.

model_lr2 <- glm(left ~ ., data = data3_trans_sel, family = binomial())
set.seed(111)
model_xgb2 <- gbm(left ~ ., data = data3_trans_sel, distribution = "bernoulli", n.trees = n.trees)

Moreover, we create a glm model based on original data in order to check if our methodology improves results.

model_lr1 <- glm(left ~ ., data = data1, family = binomial())

Comparing models performance

Final step is the comparison of all four models we have created. For each of them we make predictions on the relevant test set, i.e. we use model_lr1 and model_xgb1 to predict the output for data2 and model_lr2 and model_xgb2 to predict the output for data4.

pred_lr1 <- round(predict(model_lr1, data2, type = "response"))
pred_xgb1 <- round(predict(model_xgb1, data2, n.trees = n.trees, type = "response"))
pred_lr2 <- round(predict(model_lr2, data4_trans_sel, type = "response"))
pred_xgb2 <- round(predict(model_xgb2, data4_trans_sel, n.trees = n.trees, type = "response"))

The performance of the models may then be evaluated based on confusion matrices with relative percentages obtained via the confusion_matrix function:

confusion_matrix <- function(y_true, y_pred) {
  cm <- data.frame(pred_0 = c(sum(y_true==0 & y_pred==0)/sum(y_true==0),
                              sum(y_true==1 & y_pred==0)/sum(y_true==1)),
                   pred_1 = c(sum(y_true==0 & y_pred==1)/sum(y_true==0),
                              sum(y_true==1 & y_pred==1)/sum(y_true==1)))
  cm <- apply(cm, MARGIN = 2, function(x) round(x, 2))
  rownames(cm) <- c("actual_0", "actual_1")
  cm
}

GBM models give the following results:

  predicted 0 predicted 1
actual 0 0.97 0.03
actual 1 0.09 0.91
  predicted 0 predicted 1
actual 0 0.93 0.07
actual 1 0.27 0.73

The model trained on original data has higher predictive power - feature transformations have caused the loss of valuable information which was apparently used by the first model. However, within logistic regression models the significant improvement can be observed:

  predicted 0 predicted 1
actual 0 0.92 0.08
actual 1 0.63 0.37
  predicted 0 predicted 1
actual 0 0.93 0.07
actual 1 0.27 0.73

The initial logistic regression model has difficulties with “true negatives” - for employees that actually quit it very often predicts the opposite. On the other hand, model_lr2 which was trained on the set of features modified by the SAFE algorithm has much better accuracy.