`step_YeoJohnson`

creates a *specification* of a
recipe step that will transform data using a simple Yeo-Johnson
transformation.

```
step_YeoJohnson(
recipe,
...,
role = NA,
trained = FALSE,
lambdas = NULL,
limits = c(-5, 5),
num_unique = 5,
na_rm = TRUE,
skip = FALSE,
id = rand_id("YeoJohnson")
)
```

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
variables are affected by the step. See |

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. |

lambdas | A numeric vector of transformation values. This
is |

limits | A length 2 numeric vector defining the range to compute the transformation parameter lambda. |

num_unique | An integer where data that have less possible values will not be evaluated for a transformation. |

na_rm | A logical value indicating whether |

skip | A logical. Should the step be skipped when the
recipe is baked by |

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

An updated version of `recipe`

with the new step
added to the sequence of existing steps (if any).

The Yeo-Johnson transformation is very similar to the Box-Cox but does not require the input variables to be strictly positive. In the package, the partial log-likelihood function is directly optimized within a reasonable set of transformation values (which can be changed by the user).

This transformation is typically done on the outcome variable
using the residuals for a statistical model (such as ordinary
least squares). Here, a simple null model (intercept only) is
used to apply the transformation to the *predictor*
variables individually. This can have the effect of making the
variable distributions more symmetric.

If the transformation parameters are estimated to be very
closed to the bounds, or if the optimization fails, a value of
`NA`

is used and no transformation is applied.

When you `tidy()`

this step, a tibble with columns `terms`

(the
selectors or variables selected) and `value`

(the
lambda estimate) is returned.

Yeo, I. K., and Johnson, R. A. (2000). A new family of power
transformations to improve normality or symmetry. *Biometrika*.

```
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)
yj_transform <- step_YeoJohnson(rec, all_numeric())
yj_estimates <- prep(yj_transform, training = biomass_tr)
yj_te <- bake(yj_estimates, biomass_te)
plot(density(biomass_te$sulfur), main = "before")
plot(density(yj_te$sulfur), main = "after")
tidy(yj_transform, number = 1)
#> # A tibble: 1 x 3
#> terms value id
#> <chr> <dbl> <chr>
#> 1 all_numeric() NA YeoJohnson_MuJac
tidy(yj_estimates, number = 1)
#> # A tibble: 6 x 3
#> terms value id
#> <chr> <dbl> <chr>
#> 1 carbon -0.0225 YeoJohnson_MuJac
#> 2 hydrogen 2.10 YeoJohnson_MuJac
#> 3 oxygen 1.78 YeoJohnson_MuJac
#> 4 nitrogen -0.830 YeoJohnson_MuJac
#> 5 sulfur -4.09 YeoJohnson_MuJac
#> 6 HHV -0.388 YeoJohnson_MuJac
```