`tune_grid()`

computes a set of performance metrics (e.g. accuracy or RMSE)
for a pre-defined set of tuning parameters that correspond to a model or
recipe across one or more resamples of the data.

## Usage

```
tune_grid(object, ...)
# S3 method for model_spec
tune_grid(
object,
preprocessor,
resamples,
...,
param_info = NULL,
grid = 10,
metrics = NULL,
eval_time = NULL,
control = control_grid()
)
# S3 method for workflow
tune_grid(
object,
resamples,
...,
param_info = NULL,
grid = 10,
metrics = NULL,
eval_time = NULL,
control = control_grid()
)
```

## Arguments

- object
A

`parsnip`

model specification or an unfitted workflow(). No tuning parameters are allowed; if arguments have been marked with tune(), their values must be finalized.- ...
Not currently used.

- preprocessor
A traditional model formula or a recipe created using

`recipes::recipe()`

.- resamples
An

`rset`

resampling object created from an`rsample`

function, such as`rsample::vfold_cv()`

.- param_info
A

`dials::parameters()`

object or`NULL`

. If none is given, a parameters set is derived from other arguments. Passing this argument can be useful when parameter ranges need to be customized.- grid
A data frame of tuning combinations or a positive integer. The data frame should have columns for each parameter being tuned and rows for tuning parameter candidates. An integer denotes the number of candidate parameter sets to be created automatically.

- metrics
A

`yardstick::metric_set()`

, or`NULL`

to compute a standard set of metrics.- eval_time
A numeric vector of time points where dynamic event time metrics should be computed (e.g. the time-dependent ROC curve, etc). The values must be non-negative and should probably be no greater than the largest event time in the training set (See Details below).

- control
An object used to modify the tuning process, likely created by

`control_grid()`

.

## Value

An updated version of `resamples`

with extra list columns for `.metrics`

and
`.notes`

(optional columns are `.predictions`

and `.extracts`

). `.notes`

contains warnings and errors that occur during execution.

## Details

Suppose there are *m* tuning parameter combinations. `tune_grid()`

may not
require all *m* model/recipe fits across each resample. For example:

In cases where a single model fit can be used to make predictions for different parameter values in the grid, only one fit is used. For example, for some boosted trees, if 100 iterations of boosting are requested, the model object for 100 iterations can be used to make predictions on iterations less than 100 (if all other parameters are equal).

When the model is being tuned in conjunction with pre-processing and/or post-processing parameters, the minimum number of fits are used. For example, if the number of PCA components in a recipe step are being tuned over three values (along with model tuning parameters), only three recipes are trained. The alternative would be to re-train the same recipe multiple times for each model tuning parameter.

tune supports parallel processing with the future package. To execute
the resampling iterations in parallel, specify a plan with
future first. The `allow_par`

argument can be used to avoid parallelism.

For the most part, warnings generated during training are shown as they occur
and are associated with a specific resample when
`control_grid(verbose = TRUE)`

. They are (usually) not aggregated until the
end of processing.

## Parameter Grids

If no tuning grid is provided, a semi-random grid (via
`dials::grid_latin_hypercube()`

) is created with 10 candidate parameter
combinations.

When provided, the grid should have column names for each parameter and
these should be named by the parameter name or `id`

. For example, if a
parameter is marked for optimization using `penalty = tune()`

, there should
be a column named `penalty`

. If the optional identifier is used, such as
`penalty = tune(id = 'lambda')`

, then the corresponding column name should
be `lambda`

.

In some cases, the tuning parameter values depend on the dimensions of the
data. For example, `mtry`

in random forest models depends on the number of
predictors. In this case, the default tuning parameter object requires an
upper range. `dials::finalize()`

can be used to derive the data-dependent
parameters. Otherwise, a parameter set can be created (via
`dials::parameters()`

) and the `dials`

`update()`

function can be used to
change the values. This updated parameter set can be passed to the function
via the `param_info`

argument.

## Performance Metrics

To use your own performance metrics, the `yardstick::metric_set()`

function
can be used to pick what should be measured for each model. If multiple
metrics are desired, they can be bundled. For example, to estimate the area
under the ROC curve as well as the sensitivity and specificity (under the
typical probability cutoff of 0.50), the `metrics`

argument could be given:

```
metrics = metric_set(roc_auc, sens, spec)
```

Each metric is calculated for each candidate model.

If no metric set is provided, one is created:

For regression models, the root mean squared error and coefficient of determination are computed.

For classification, the area under the ROC curve and overall accuracy are computed.

Note that the metrics also determine what type of predictions are estimated during tuning. For example, in a classification problem, if metrics are used that are all associated with hard class predictions, the classification probabilities are not created.

The out-of-sample estimates of these metrics are contained in a list column
called `.metrics`

. This tibble contains a row for each metric and columns
for the value, the estimator type, and so on.

`collect_metrics()`

can be used for these objects to collapse the results
over the resampled (to obtain the final resampling estimates per tuning
parameter combination).

## Obtaining Predictions

When `control_grid(save_pred = TRUE)`

, the output tibble contains a list
column called `.predictions`

that has the out-of-sample predictions for each
parameter combination in the grid and each fold (which can be very large).

The elements of the tibble are tibbles with columns for the tuning
parameters, the row number from the original data object (`.row`

), the
outcome data (with the same name(s) of the original data), and any columns
created by the predictions. For example, for simple regression problems, this
function generates a column called `.pred`

and so on. As noted above, the
prediction columns that are returned are determined by the type of metric(s)
requested.

This list column can be `unnested`

using `tidyr::unnest()`

or using the
convenience function `collect_predictions()`

.

## Extracting Information

The `extract`

control option will result in an additional function to be
returned called `.extracts`

. This is a list column that has tibbles
containing the results of the user's function for each tuning parameter
combination. This can enable returning each model and/or recipe object that
is created during resampling. Note that this could result in a large return
object, depending on what is returned.

The control function contains an option (`extract`

) that can be used to
retain any model or recipe that was created within the resamples. This
argument should be a function with a single argument. The value of the
argument that is given to the function in each resample is a workflow
object (see `workflows::workflow()`

for more information). Several
helper functions can be used to easily pull out the preprocessing
and/or model information from the workflow, such as
`extract_preprocessor()`

and
`extract_fit_parsnip()`

.

As an example, if there is interest in getting each parsnip model fit back, one could use:

```
extract = function (x) extract_fit_parsnip(x)
```

Note that the function given to the `extract`

argument is evaluated on
every model that is *fit* (as opposed to every model that is *evaluated*).
As noted above, in some cases, model predictions can be derived for
sub-models so that, in these cases, not every row in the tuning parameter
grid has a separate R object associated with it.

## Case Weights

Some models can utilize case weights during training. tidymodels currently supports two types of case weights: importance weights (doubles) and frequency weights (integers). Frequency weights are used during model fitting and evaluation, whereas importance weights are only used during fitting.

To know if your model is capable of using case weights, create a model spec
and test it using `parsnip::case_weights_allowed()`

.

To use them, you will need a numeric column in your data set that has been
passed through either `hardhat:: importance_weights()`

or
`hardhat::frequency_weights()`

.

For functions such as `fit_resamples()`

and the `tune_*()`

functions, the
model must be contained inside of a `workflows::workflow()`

. To declare that
case weights are used, invoke `workflows::add_case_weights()`

with the
corresponding (unquoted) column name.

From there, the packages will appropriately handle the weights during model fitting and (if appropriate) performance estimation.

## Censored Regression Models

Three types of metrics can be used to assess the quality of censored regression models:

static: the prediction is independent of time.

dynamic: the prediction is a time-specific probability (e.g., survival probability) and is measured at one or more particular times.

integrated: same as the dynamic metric but returns the integral of the different metrics from each time point.

Which metrics are chosen by the user affects how many evaluation times should be specified. For example:

```
# Needs no `eval_time` value
metric_set(concordance_survival)
# Needs at least one `eval_time`
metric_set(brier_survival)
metric_set(brier_survival, concordance_survival)
# Needs at least two eval_time` values
metric_set(brier_survival_integrated, concordance_survival)
metric_set(brier_survival_integrated, concordance_survival)
metric_set(brier_survival_integrated, concordance_survival, brier_survival)
```

Values of `eval_time`

should be less than the largest observed event
time in the training data. For many non-parametric models, the results beyond
the largest time corresponding to an event are constant (or `NA`

).

## Examples

```
library(recipes)
library(rsample)
library(parsnip)
library(workflows)
library(ggplot2)
# ---------------------------------------------------------------------------
set.seed(6735)
folds <- vfold_cv(mtcars, v = 5)
# ---------------------------------------------------------------------------
# tuning recipe parameters:
spline_rec <-
recipe(mpg ~ ., data = mtcars) %>%
step_ns(disp, deg_free = tune("disp")) %>%
step_ns(wt, deg_free = tune("wt"))
lin_mod <-
linear_reg() %>%
set_engine("lm")
# manually create a grid
spline_grid <- expand.grid(disp = 2:5, wt = 2:5)
# Warnings will occur from making spline terms on the holdout data that are
# extrapolations.
spline_res <-
tune_grid(lin_mod, spline_rec, resamples = folds, grid = spline_grid)
spline_res
#> # Tuning results
#> # 5-fold cross-validation
#> # A tibble: 5 × 4
#> splits id .metrics .notes
#> <list> <chr> <list> <list>
#> 1 <split [25/7]> Fold1 <tibble [32 × 6]> <tibble [0 × 3]>
#> 2 <split [25/7]> Fold2 <tibble [32 × 6]> <tibble [0 × 3]>
#> 3 <split [26/6]> Fold3 <tibble [32 × 6]> <tibble [0 × 3]>
#> 4 <split [26/6]> Fold4 <tibble [32 × 6]> <tibble [0 × 3]>
#> 5 <split [26/6]> Fold5 <tibble [32 × 6]> <tibble [0 × 3]>
show_best(spline_res, metric = "rmse")
#> # A tibble: 5 × 8
#> disp wt .metric .estimator mean n std_err .config
#> <int> <int> <chr> <chr> <dbl> <int> <dbl> <chr>
#> 1 3 2 rmse standard 2.54 5 0.207 Preprocessor02_Model1
#> 2 3 3 rmse standard 2.64 5 0.234 Preprocessor06_Model1
#> 3 4 3 rmse standard 2.82 5 0.456 Preprocessor07_Model1
#> 4 4 2 rmse standard 2.93 5 0.489 Preprocessor03_Model1
#> 5 4 4 rmse standard 3.01 5 0.475 Preprocessor11_Model1
# ---------------------------------------------------------------------------
# tune model parameters only (example requires the `kernlab` package)
car_rec <-
recipe(mpg ~ ., data = mtcars) %>%
step_normalize(all_predictors())
svm_mod <-
svm_rbf(cost = tune(), rbf_sigma = tune()) %>%
set_engine("kernlab") %>%
set_mode("regression")
# Use a space-filling design with 7 points
set.seed(3254)
svm_res <- tune_grid(svm_mod, car_rec, resamples = folds, grid = 7)
svm_res
#> # Tuning results
#> # 5-fold cross-validation
#> # A tibble: 5 × 4
#> splits id .metrics .notes
#> <list> <chr> <list> <list>
#> 1 <split [25/7]> Fold1 <tibble [14 × 6]> <tibble [0 × 3]>
#> 2 <split [25/7]> Fold2 <tibble [14 × 6]> <tibble [0 × 3]>
#> 3 <split [26/6]> Fold3 <tibble [14 × 6]> <tibble [0 × 3]>
#> 4 <split [26/6]> Fold4 <tibble [14 × 6]> <tibble [0 × 3]>
#> 5 <split [26/6]> Fold5 <tibble [14 × 6]> <tibble [0 × 3]>
show_best(svm_res, metric = "rmse")
#> # A tibble: 5 × 8
#> cost rbf_sigma .metric .estimator mean n std_err .config
#> <dbl> <dbl> <chr> <chr> <dbl> <int> <dbl> <chr>
#> 1 0.304 0.117 rmse standard 3.91 5 0.652 Preprocesso…
#> 2 4.53 0.000420 rmse standard 4.13 5 0.741 Preprocesso…
#> 3 0.00247 0.00931 rmse standard 5.94 5 0.966 Preprocesso…
#> 4 23.2 0.000000684 rmse standard 5.94 5 0.967 Preprocesso…
#> 5 0.0126 0.00000239 rmse standard 5.96 5 0.970 Preprocesso…
autoplot(svm_res, metric = "rmse") +
scale_x_log10()
#> Warning: NaNs produced
#> Warning: log-10 transformation introduced infinite values.
#> Warning: Removed 12 rows containing missing values or values outside the scale
#> range (`geom_point()`).
# ---------------------------------------------------------------------------
# Using a variables preprocessor with a workflow
# Rather than supplying a preprocessor (like a recipe) and a model directly
# to `tune_grid()`, you can also wrap them up in a workflow and pass
# that along instead (note that this doesn't do any preprocessing to
# the variables, it passes them along as-is).
wf <- workflow() %>%
add_variables(outcomes = mpg, predictors = everything()) %>%
add_model(svm_mod)
set.seed(3254)
svm_res_wf <- tune_grid(wf, resamples = folds, grid = 7)
```