# Getting Started with xplainfi

    library(xplainfi)
    library(mlr3)
    library(mlr3learners)
    library(data.table)
    library(ggplot2)

xplainfi provides feature importance methods for machine learning models. It implements several approaches for measuring how much each feature contributes to model performance, with a focus on model-agnostic methods that work with any learner.

## Core Concepts

Feature importance methods in `xplainfi` address different but related questions:

- How much does each feature contribute to model performance? (Permutation Feature Importance)
- What happens when we remove features and retrain? (Leave-One-Covariate-Out)
- How do features depend on each other? (Conditional and Relative methods)

All methods share a common interface built on mlr3, making them easy to use with any task, learner, measure, and resampling strategy.

The general pattern is to call `$compute()` to calculate importance (which always re-computes), then `$importance()` to retrieve the aggregated results, with intermediate results available in `$scores()` and, if the chosen measures supports it, `$obs_loss()`.

## Basic Example

Let’s use the Friedman1 task, which provides an ideal setup for demonstrating feature importance methods with known ground truth:

    task <- tgen("friedman1")$generate(n = 300)
    learner <- lrn("regr.ranger", num.trees = 100)
    measure <- msr("regr.mse")
    resampling <- rsmp("cv", folds = 3)

The task has 300 observations with 10 features. Features `important1` through `important5` truly affect the target, while `unimportant1` through `unimportant5` are pure noise. We’ll use a random forest learner with cross-validation for more stable estimates.

The target function is: \(y = 10 * \operatorname{sin}(\pi * x_1 * x_2) + 20 * (x_3 - 0.5)^2 + 10 * x_4 + 5 * x_5 + \epsilon\)

## Permutation Feature Importance (PFI)

PFI is the most straightforward method: for each feature, we permute (shuffle) its values and measure how much model performance deteriorates. More important features cause larger performance drops when shuffled.

    pfi <- PFI$new(
        task = task,
        learner = learner,
        measure = measure,
        resampling = resampling
    )

    pfi$compute()
    pfi$importance()
    #> Key: <feature>
    #>          feature   importance
    #>           <char>        <num>
    #>  1:   important1  5.087261575
    #>  2:   important2  7.934989146
    #>  3:   important3  1.091293042
    #>  4:   important4 10.924662281
    #>  5:   important5  2.432564965
    #>  6: unimportant1 -0.024310146
    #>  7: unimportant2  0.159875469
    #>  8: unimportant3  0.023231148
    #>  9: unimportant4  0.000284637
    #> 10: unimportant5 -0.039391565

The `importance` column shows the performance difference when each feature is permuted. Higher values indicate more important features.

For more stable estimates, we can use multiple permutation iterations per resampling fold with `n_repeats`. Note that in this case “more is more”, and while there is no clear “good enough” value, setting `n_repeats` to a small value like 1 will most definitely yield unreliable results.

    pfi_stable <- PFI$new(
        task = task,
        learner = learner,
        measure = measure,
        resampling = resampling,
        n_repeats = 50
    )

    pfi_stable$compute()
    pfi_stable$importance()
    #> Key: <feature>
    #>          feature  importance
    #>           <char>       <num>
    #>  1:   important1  5.69331523
    #>  2:   important2  8.14763339
    #>  3:   important3  1.18196174
    #>  4:   important4 11.99249476
    #>  5:   important5  2.04693042
    #>  6: unimportant1 -0.01206658
    #>  7: unimportant2 -0.01718936
    #>  8: unimportant3 -0.08372375
    #>  9: unimportant4  0.08570232
    #> 10: unimportant5 -0.05251443

We can also use ratio instead of difference for the importance calculation, meaning that an unimportant feature is now expected to get an importance score of 1 rather than 0:

    pfi_stable$importance(relation = "ratio")
    #> Key: <feature>
    #>          feature importance
    #>           <char>      <num>
    #>  1:   important1  1.8377409
    #>  2:   important2  2.2040144
    #>  3:   important3  1.1752990
    #>  4:   important4  2.7584425
    #>  5:   important5  1.3009155
    #>  6: unimportant1  0.9985212
    #>  7: unimportant2  0.9970989
    #>  8: unimportant3  0.9881171
    #>  9: unimportant4  1.0123079
    #> 10: unimportant5  0.9919494

## Leave-One-Covariate-Out (LOCO)

LOCO measures importance by retraining the model without each feature and comparing performance to the full model. This shows the contribution of each feature when all other features are present.

    loco <- LOCO$new(
        task = task,
        learner = learner,
        measure = measure,
        resampling = resampling
    )

    loco$compute()
    loco$importance()
    #> Key: <feature>
    #>          feature importance
    #>           <char>      <num>
    #>  1:   important1  3.4607216
    #>  2:   important2  5.8198340
    #>  3:   important3  0.9363660
    #>  4:   important4  7.4402008
    #>  5:   important5  0.4303848
    #>  6: unimportant1 -0.3244184
    #>  7: unimportant2 -0.1915122
    #>  8: unimportant3 -0.1675775
    #>  9: unimportant4 -0.1774725
    #> 10: unimportant5 -0.4084427

LOCO is computationally expensive as it requires retraining for each feature, but provides clear interpretation: higher values mean larger performance drop when the feature is removed. However, it cannot distinguish between direct effects and indirect effects through correlated features.

## Feature Samplers

For advanced methods that account for feature dependencies, xplainfi provides different sampling strategies. While PFI uses simple permutation (marginal sampling), conditional samplers can preserve feature relationships.

Let’s demonstrate conditional sampling using adversarial random rorests (ARF), which preserves relationships between features when sampling:

    arf_sampler <- ConditionalARFSampler$new(task)

    sample_data <- task$data(rows = 1:5)
    sample_data[, .(important1, important2)]
    #>    important1  important2
    #>         <num>       <num>
    #> 1:  0.2875775 0.784575267
    #> 2:  0.7883051 0.009429905
    #> 3:  0.4089769 0.779065883
    #> 4:  0.8830174 0.729390652
    #> 5:  0.9404673 0.630131853

Now we’ll conditionally sample the `important1` feature given the values of `important2` and `important3`:

    sampled_conditional <- arf_sampler$sample_newdata(
        feature = "important1",
        newdata = sample_data,
        conditioning_set = c("important2", "important3")
    )

    sample_data[, .(important1, important2, important3)]
    #>    important1  important2 important3
    #>         <num>       <num>      <num>
    #> 1:  0.2875775 0.784575267  0.2372297
    #> 2:  0.7883051 0.009429905  0.6864904
    #> 3:  0.4089769 0.779065883  0.2258184
    #> 4:  0.8830174 0.729390652  0.3184946
    #> 5:  0.9404673 0.630131853  0.1739838
    sampled_conditional[, .(important1, important2, important3)]
    #>    important1  important2 important3
    #>         <num>       <num>      <num>
    #> 1: 0.26312558 0.784575267  0.2372297
    #> 2: 0.49047991 0.009429905  0.6864904
    #> 3: 0.08545050 0.779065883  0.2258184
    #> 4: 0.28119269 0.729390652  0.3184946
    #> 5: 0.06460809 0.630131853  0.1739838

This conditional sampling is essential for methods like CFI and RFI that need to preserve feature dependencies. See the perturbation-importance article for detailed comparisons and `vignette("feature-samplers")` for more details on implemented samplers.

## Detailed Scoring Information

All methods store detailed scoring information from each resampling iteration for further analysis. Let’s examine the structure of PFI’s detailed scores:

    pfi$scores() |>
        head(10) |>
        knitr::kable(digits = 4, caption = "Detailed PFI scores (first 10 rows)")

Detailed PFI scores (first 10 rows)
feature | iter_rsmp | iter_repeat | regr.mse_baseline | regr.mse_post | importance
--- | --- | --- | --- | --- | ---
important1 | 1 | 1 | 4.3639 | 9.1892 | 4.8253
important2 | 1 | 1 | 4.3639 | 10.7456 | 6.3817
important3 | 1 | 1 | 4.3639 | 5.0741 | 0.7102
important4 | 1 | 1 | 4.3639 | 15.5004 | 11.1365
important5 | 1 | 1 | 4.3639 | 6.4515 | 2.0876
unimportant1 | 1 | 1 | 4.3639 | 4.3224 | -0.0415
unimportant2 | 1 | 1 | 4.3639 | 4.5137 | 0.1498
unimportant3 | 1 | 1 | 4.3639 | 4.4032 | 0.0393
unimportant4 | 1 | 1 | 4.3639 | 4.2489 | -0.1150
unimportant5 | 1 | 1 | 4.3639 | 4.2589 | -0.1050

We can also summarize the scoring structure:

    pfi$scores()[, .(
        features = uniqueN(feature),
        resampling_folds = uniqueN(iter_rsmp),
        permutation_iters = uniqueN(iter_repeat),
        total_scores = .N
    )]
    #>    features resampling_folds permutation_iters total_scores
    #>       <int>            <int>             <int>        <int>
    #> 1:       10                3                 1           30

So `$importance()` always gives us the aggregated importances across multiple resampling- and permutation-/refitting iterations, whereas `$scores()` gives you the individual scores as calculated by the supplied `measures` and the corresponding importance calculated from the difference of these scores by default.

Analogously to `$importance()`, you can also use `relation = "ratio"` here:

    pfi$scores(relation = "ratio") |>
        head(10) |>
        knitr::kable(digits = 4, caption = "PFI scores using the ratio (first 10 rows)")

PFI scores using the ratio (first 10 rows)
feature | iter_rsmp | iter_repeat | regr.mse_baseline | regr.mse_post | importance
--- | --- | --- | --- | --- | ---
important1 | 1 | 1 | 4.3639 | 9.1892 | 2.1057
important2 | 1 | 1 | 4.3639 | 10.7456 | 2.4624
important3 | 1 | 1 | 4.3639 | 5.0741 | 1.1627
important4 | 1 | 1 | 4.3639 | 15.5004 | 3.5520
important5 | 1 | 1 | 4.3639 | 6.4515 | 1.4784
unimportant1 | 1 | 1 | 4.3639 | 4.3224 | 0.9905
unimportant2 | 1 | 1 | 4.3639 | 4.5137 | 1.0343
unimportant3 | 1 | 1 | 4.3639 | 4.4032 | 1.0090
unimportant4 | 1 | 1 | 4.3639 | 4.2489 | 0.9736
unimportant5 | 1 | 1 | 4.3639 | 4.2589 | 0.9759

## Observation-wise losses and importances

For methods where importances are calculated based on observation-level comparisons and with decomposable measures, we can also retrieve observation-level information with `$obs_loss()`, which works analogously to `$scores()` and `$importances()` but even more detailed:

    pfi$obs_loss()
    #>            feature iter_rsmp iter_repeat row_ids loss_baseline   loss_post
    #>             <char>     <int>       <int>   <int>         <num>       <num>
    #>    1:   important1         1           1       1    4.27303383  0.93888898
    #>    2:   important1         1           1       9    0.94121187  0.11011124
    #>    3:   important1         1           1      11    0.01545140 16.00963101
    #>    4:   important1         1           1      12    0.13814810  0.29308440
    #>    5:   important1         1           1      15   11.87605546 46.83609001
    #>   ---
    #> 2996: unimportant5         3           1     290    7.97305649  8.01220503
    #> 2997: unimportant5         3           1     294    1.06406580  0.92691286
    #> 2998: unimportant5         3           1     295   10.92947732 10.76761731
    #> 2999: unimportant5         3           1     296    0.03164589  0.08982416
    #> 3000: unimportant5         3           1     298   14.52226394 14.52226394
    #>       obs_importance
    #>                <num>
    #>    1:    -3.33414486
    #>    2:    -0.83110062
    #>    3:    15.99417961
    #>    4:     0.15493629
    #>    5:    34.96003455
    #>   ---
    #> 2996:     0.03914854
    #> 2997:    -0.13715294
    #> 2998:    -0.16186002
    #> 2999:     0.05817826
    #> 3000:     0.00000000

Since we computed PFI using the mean squared error (`msr("regr.mse")`), we can use the associated `Measure$obs_loss()`, the squared error.
In the resulting table we see

- `loss_baseline`: The loss (squared error) for the baseline model before permutation
- `loss_post`: The loss for this observation after permutation (or in the case of `LOCO`, after refit)
- `obs_importance`: The difference (or ratio if `relation = "ratio"`) of the the two losses

Note that not all measures have a `Measure$obs_loss()`: Some measures like `msr("classif.auc")` are not decomposable, so observation-wise loss values are not available.
In other cases, the corresponding `obs_loss()` is just not yet implemented in `mlr3measures`, but will likely be in the future.

## Parallelization

Both PFI/CFI/RFI and LOCO/WVIM support parallel execution to speed up computation when working with multiple features or expensive learners. The parallelization follows mlr3’s approach, allowing users to choose between `mirai` and `future` backends.

### Example with future

The `future` package provides a simple interface for parallel and distributed computing:

    library(future)
    plan("multisession", workers = 2)

    # PFI with parallelization across features
    pfi_parallel = PFI$new(
        task,
        learner = lrn("regr.ranger"),
        measure = msr("regr.mse"),
        n_repeats = 10
    )
    pfi_parallel$compute()
    pfi_parallel$importance()

    # LOCO with parallelization (uses mlr3fselect internally)
    loco_parallel = LOCO$new(
        task,
        learner = lrn("regr.ranger"),
        measure = msr("regr.mse")
    )
    loco_parallel$compute()
    loco_parallel$importance()

### Example with mirai

The `mirai` package offers a modern alternative for parallel computing:

    library(mirai)
    daemons(n = 2)

    # Same PFI/LOCO code works with mirai backend
    pfi_parallel = PFI$new(
        task,
        learner = lrn("regr.ranger"),
        measure = msr("regr.mse"),
        n_repeats = 10
    )
    pfi_parallel$compute()
    pfi_parallel$importance()

    # Clean up daemons when done
    daemons(0)