B-Spline Basis Functions

step_bs creates a specification of a recipe step that will create new columns that are basis expansions of variables using B-splines.

Usage

step_bs(
  recipe,
  ...,
  role = "predictor",
  trained = FALSE,
  deg_free = NULL,
  degree = 3,
  objects = NULL,
  options = list(),
  skip = FALSE,
  id = rand_id("bs")
)

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.
role: For model terms created by this step, what analysis role should they be assigned? By default, the new columns created by this step from the original variables will be used as predictors in a model.
trained: A logical to indicate if the quantities for preprocessing have been estimated.
deg_free: The degrees of freedom for the spline. As the degrees of freedom for a spline increase, more flexible and complex curves can be generated. When a single degree of freedom is used, the result is a rescaled version of the original data.
degree: Degree of polynomial spline (integer).
objects: A list of splines::bs() objects created once the step has been trained.
options: A list of options for splines::bs() which should not include x, degree, or df.
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.

Value

An updated version of recipe with the new step added to the sequence of any existing operations.

Details

step_bs can create new features from a single variable that enable fitting routines to model this variable in a nonlinear manner. The extent of the possible nonlinearity is determined by the df, degree, or knot arguments of splines::bs(). The original variables are removed from the data and new columns are added. The naming convention for the new variables is varname_bs_1 and so on.

Tidying

When you tidy() this step, a tibble with column terms (the columns that will be affected) is returned.

Examples

library(modeldata)
data(biomass)

biomass_tr <- biomass[biomass$dataset == "Training",]
biomass_te <- biomass[biomass$dataset == "Testing",]

rec <- recipe(HHV ~ carbon + hydrogen + oxygen + nitrogen + sulfur,
              data = biomass_tr)

with_splines <- rec %>%
  step_bs(carbon, hydrogen)
with_splines <- prep(with_splines, training = biomass_tr)

expanded <- bake(with_splines, biomass_te)
expanded
#> # A tibble: 80 × 10
#>    oxygen nitrogen sulfur   HHV carbon_bs_1 carbon_bs_2 carbon_bs_3
#>     <dbl>    <dbl>  <dbl> <dbl>       <dbl>       <dbl>       <dbl>
#>  1   47.2     0.3    0.22  18.3       0.437      0.273      0.0568 
#>  2   48.1     2.85   0.34  17.6       0.444      0.236      0.0417 
#>  3   49.1     2.4    0.3   17.2       0.444      0.229      0.0394 
#>  4   37.3     1.8    0.5   18.9       0.437      0.273      0.0571 
#>  5   42.8     0.2    0     20.5       0.427      0.301      0.0707 
#>  6   41.7     0.7    0.2   18.5       0.442      0.248      0.0465 
#>  7   54.1     1.19   0.51  15.1       0.440      0.184      0.0256 
#>  8   33.8     0.95   0.2   16.2       0.444      0.222      0.0369 
#>  9   31.1     0.14   4.9   11.1       0.359      0.0771     0.00552
#> 10   23.7     4.63   1.05  10.8       0.338      0.0643     0.00408
#> # … with 70 more rows, and 3 more variables: hydrogen_bs_1 <dbl>,
#> #   hydrogen_bs_2 <dbl>, hydrogen_bs_3 <dbl>