step_classdist_shrunken: Compute shrunken centroid distances for classification models

Description

step_classdist_shrunken creates a specification of a recipe step that will convert numeric data into Euclidean distance to the regularized class centroid. This is done for each value of a categorical class variable.

Usage

step_classdist_shrunken(
  recipe,
  ...,
  class = NULL,
  role = NA,
  trained = FALSE,
  threshold = 1/2,
  sd_offset = 1/2,
  log = TRUE,
  prefix = "classdist_",
  keep_original_cols = TRUE,
  objects = NULL,
  skip = FALSE,
  id = rand_id("classdist_shrunken")
)

Arguments

recipe: A recipe object. The step will be added to the sequence of operations for this recipe.
...: One or more selector functions to choose variables for this step. See selections() for more details.
class: A single character string that specifies a single categorical variable to be used as the class.
role: Not used by this step since no new variables are created.
trained: A logical to indicate if the quantities for preprocessing have been estimated.
threshold: A regularization parameter between zero and one. Zero means that no regularization is used and one means that centroids should be shrunk to the global centroid.
sd_offset: A value between zero and one for the quantile that should be used to stabilize the pooled standard deviation.
log: A logical: should the distances be transformed by the natural log function?
prefix: A character string for the prefix of the resulting new variables. See notes below.
keep_original_cols: A logical to keep the original variables in the output. Defaults to TRUE.
objects: Statistics are stored here once this step has been trained by prep().
skip: A logical. Should the step be skipped when the recipe is baked by bake()? While all operations are baked when prep() is run, some operations may not be able to be conducted on new data (e.g. processing the outcome variable(s)). Care should be taken when using skip = TRUE as it may affect the computations for subsequent operations.
id: A character string that is unique to this step to identify it.

Tidying

When you tidy() this step, a tibble is returned with columns terms, value, class, type, threshold , and id:

terms: character, the selectors or variables selected
value: numeric, the centroid
class: character, name of class variable
type: character, has values "global", "by_class", and "shrunken"
threshold: numeric, value of threshold
id: character, id of this step

The first two types of centroids are in the original units while the last has been standardized.

Case weights

This step performs an supervised operation that can utilize case weights. As a result, case weights are used with frequency weights as well as importance weights. For more information,, see the documentation in case_weights and the examples on tidymodels.org.

Details

Class-specific centroids are the multivariate averages of each predictor using the data from each class in the training set. When pre-processing a new data point, this step computes the distance from the new point to each of the class centroids. These distance features can be very effective at capturing linear class boundaries. For this reason, they can be useful to add to an existing predictor set used within a nonlinear model. If the true boundary is actually linear, the model will have an easier time learning the training data patterns.

Shrunken centroids use a form of regularization where the class-specific centroids are contracted to the overall class-independent centroid. If a predictor is uninformative, shrinking it may move it entirely to the overall centroid. This has the effect of removing that predictor's effect on the new distance features. However, it may not move all of the class-specific features to the center in many cases. This means that some features will only affect the classification of specific classes.

The threshold parameter can be used to optimized how much regularization should be used.

step_classdist_shrunken will create a new column for every unique value of the class variable. The resulting variables will not replace the original values and, by default, have the prefix classdist_. The naming format can be changed using the prefix argument.

References

Tibshirani, R., Hastie, T., Narasimhan, B., & Chu, G. (2002). Diagnosis of multiple cancer types by shrunken centroids of gene expression. Proceedings of the National Academy of Sciences, 99(10), 6567-6572.

Examples

Run this code

data(penguins, package = "modeldata")
penguins <- penguins[vctrs::vec_detect_complete(penguins), ]
penguins$island <- NULL
penguins$sex <- NULL

# define naming convention
rec <- recipe(species ~ ., data = penguins) %>%
  step_classdist_shrunken(all_numeric_predictors(),
    class = "species",
    threshold = 1 / 4, prefix = "centroid_"
  )

# default naming
rec <- recipe(species ~ ., data = penguins) %>%
  step_classdist_shrunken(all_numeric_predictors(),
    class = "species",
    threshold = 3 / 4
  )

rec_dists <- prep(rec, training = penguins)

dists_to_species <- bake(rec_dists, new_data = penguins)
## on log scale:
dist_cols <- grep("classdist", names(dists_to_species), value = TRUE)
dists_to_species[, c("species", dist_cols)]

tidy(rec, number = 1)
tidy(rec_dists, number = 1)

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