recipes (version 0.1.7)

step_downsample: Down-Sample a Data Set Based on a Factor Variable

Description

step_downsample creates a specification of a recipe step that will remove rows of a data set to make the occurrence of levels in a specific factor level equal.

Usage

step_downsample(recipe, ..., under_ratio = 1, ratio = NA, role = NA,
  trained = FALSE, column = NULL, target = NA, skip = TRUE,
  seed = sample.int(10^5, 1), id = rand_id("downsample"))
# S3 method for step_downsample
tidy(x, ...)

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 which variable is used to sample the data. See selections() for more details. The selection should result in single factor variable. For the tidy method, these are not currently used.

under_ratio

A numeric value for the ratio of the minority-to-majority frequencies. The default value (1) means that all other levels are sampled down to have the same frequency as the least occurring level. A value of 2 would mean that the majority levels will have (at most) (approximately) twice as many rows than the minority level.

ratio

Depracated argument; same as under_ratio

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.

column

A character string of the variable name that will be populated (eventually) by the ... selectors.

target

An integer that will be used to subsample. This should not be set by the user and will be populated by prep.

skip

A logical. Should the step be skipped when the recipe is baked by bake.recipe()? While all operations are baked when prep.recipe() 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

seed

An integer that will be used as the seed when downsampling.

id

A character string that is unique to this step to identify it.

x

A step_downsample object.

Value

An updated version of recipe with the new step added to the sequence of existing steps (if any). For the tidy method, a tibble with columns terms which is the variable used to sample.

Details

Down-sampling is intended to be performed on the training set alone. For this reason, the default is skip = TRUE. It is advisable to use prep(recipe, retain = TRUE) when preparing the recipe; in this way juice() can be used to obtain the down-sampled version of the data.

If there are missing values in the factor variable that is used to define the sampling, missing data are selected at random in the same way that the other factor levels are sampled. Missing values are not used to determine the amount of data in the minority level

For any data with factor levels occurring with the same frequency as the minority level, all data will be retained.

All columns in the data are sampled and returned by juice() and bake().

Keep in mind that the location of down-sampling in the step may have effects. For example, if centering and scaling, it is not clear whether those operations should be conducted before or after rows are removed.

When used in modeling, users should strongly consider using the option skip = TRUE so that the extra sampling is not conducted outside of the training set.

Examples

# NOT RUN {
data(okc)

sort(table(okc$diet, useNA = "always"))

ds_rec <- recipe( ~ ., data = okc) %>%
  step_downsample(diet) %>%
  prep(training = okc, retain = TRUE)

table(juice(ds_rec)$diet, useNA = "always")

# since `skip` defaults to TRUE, baking the step has no effect
baked_okc <- bake(ds_rec, new_data = okc)
table(baked_okc$diet, useNA = "always")
# }