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Conditional Feature Importance with Generative Modeling Using Adversarial Random Forests
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Proposes cARFi, a model-agnostic conditional feature importance measure that samples on-manifold feature values from adversarial random forest (ARF) conditional distributions, with tests for nonzero importance.
Problem
Measuring conditional (and relative) feature importance for a fitted model requires sampling feature values from conditional distributions, which is hard for mixed tabular data; existing methods (SAGE, CS, CPI with knockoffs) are costly, lack flexible conditioning, or lack valid inference.
Approach
Use an ARF, a fast tree-based generative model fitted once on features only, to derive conditional densities for arbitrary conditioning sets without refitting, sample R replicates per instance for stable loss differences, and tune minimum node size to interpolate between marginal and conditional importance; asymptotic normality gives paired t-tests.
Key results
- In the proof of concept, one-sided paired t-tests at alpha=0.05 control type I error near 5% at effect size 0 and reach 100% power at larger effect sizes for most learners.
- In the mixed-data DAG simulation, cARFi with R=20 keeps type I error controlled for unimportant features and shows good power, improving on single-sample CPI knockoffs especially at small sample sizes.
- cARFi is reported to be faster than generating sequential knockoffs, with ARF sampling much faster at R=20 (runtime in supplement).
Measured effects reported in the paper
| What was changed | Where | Measured | Effect |
|---|---|---|---|
| Use cARFi to test feature importance at zero effect size | cARFi with neural network, support vector machine, random forest, or linear model · Proof of concept, effect size 0 | rejection rate: 5% | No clear effect |
| Use cARFi at positive effect sizes | cARFi with neural network, support vector machine, random forest, or linear model · Proof of concept, positive effect sizes | power: 100% power | Helped |
| Use cARFi with a linear model on nonlinear data | cARFi · Proof of concept, nonlinear data | power | Mixed |
| Use minimum leaf size 20 (ablation) | cARFi · Proof of concept | type I error rate and power: 5%; 100% | Helped |
| Vary minimum leaf size to very small values (2, 5, or 10) (ablation) | cARFi · Proof of concept | type I error rate and power | Mixed |
| Vary minimum leaf size to large values (50 or 100) (ablation) | cARFi · Proof of concept | type I error rate and power | Mixed |
| Increase generated replicates from R = 1 to R = 20 (ablation) | cARFi · Mixed data simulation | power and type I error stability | Helped |
| Condition on all other features rather than use marginal importance | cARFi · Impact of conditioning set, simulated DAG | feature importance | Helped |
| Condition cARFi for X3 on X1, then X2, or both | cARFi · Impact of conditioning set, simulated DAG | feature importance for X3 | Hurt |
| Condition cARFi for X3 on X5 | cARFi · Impact of conditioning set, simulated DAG | feature importance for X3 | Hurt |
| Replace marginal PFI with cARFi conditioned on all other features | cARFi · Bike-sharing dataset | feature importance | Mixed |
| Condition only on Workday | cARFi · Bike-sharing dataset | feature importance for Hour and Temperature | Helped |
Surprising findings
- Very deep trees (small minimum leaf size) give conservative tests, while shallow trees inflate type I error, so the tree-depth parameter acts as an in-between marginal-conditional knob.
Limitations
- Asymptotic normality guaranteed only for models with finite VC dimension; SVM results are empirical
- Uncertainty in the ARF conditional distributions and the learned model is ignored in the statistical tests
- Evaluation is limited to simulations and one real dataset; no sensitivity analysis over sample size, dimension, or hyperparameters
Experimental setup
Datasets: simulated Gaussian data simulated mixed categorical/Gaussian DAG data bike-sharing dataset
Benchmarks: simulated linear and nonlinear settings (proof of concept) mixed data DAG simulation bike-sharing dataset
Metrics: type I error rate statistical power feature importance values runtime
Baselines: CPI with sequential knockoffs CPI with Gaussian knockoffs conditional subgroups (CS) SAGE PFI KernelSHAP
Builds on: adversarial random forest conditional predictive impact permutation feature importance model-X knockoffs SAGE
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