Interpretability: What Did the Model Actually Learn?

You've been here before. Your web app is slow, and Chrome DevTools tells you exactly which function is eating up 342ms on the main thread. You don't guess. You profile. Interpretability is profiling for ML models. Instead of asking "what's slow?", you're asking "what's driving this prediction?"

DevTools for Your Model

The Chrome Performance tab doesn't just tell you "your page is slow." It tells you exactly which function, in which file, at which call stack is responsible. SHAP and LIME do the same for model predictions: they tell you exactly which input feature, by how much, pushed the prediction in which direction.

Feature Importance: The Simple Version

The most intuitive approach: remove each feature one at a time and see how much the prediction changes.

import * as tf from '@tensorflow/tfjs';

// Simple permutation-based feature importance
async function featureImportance(
model: tf.LayersModel,
input: number[],
featureNames: string[]
): Promise<Array<{ feature: string; importance: number }>> {
const inputTensor = tf.tensor2d([input]);
const basePrediction = (model.predict(inputTensor) as tf.Tensor).dataSync()[0];

const importances: Array<{ feature: string; importance: number }> = [];

for (let i = 0; i < input.length; i++) {
  // Zero out this feature
  const perturbed = [...input];
  perturbed[i] = 0;

  const perturbedTensor = tf.tensor2d([perturbed]);
  const newPrediction = (model.predict(perturbedTensor) as tf.Tensor).dataSync()[0];

  importances.push({
    feature: featureNames[i],
    importance: Math.abs(basePrediction - newPrediction),
  });

  perturbedTensor.dispose();
}

// Sort by importance (most impactful first)
importances.sort((a, b) => b.importance - a.importance);

inputTensor.dispose();
return importances;
}

// Usage
const features = ['income', 'age', 'zipCode', 'creditScore', 'employmentYears'];
const applicant = [55000, 34, 90210, 720, 5];

const result = await featureImportance(model, applicant, features);
// [
//   { feature: 'creditScore', importance: 0.42 },
//   { feature: 'income', importance: 0.31 },
//   { feature: 'employmentYears', importance: 0.12 },
//   { feature: 'age', importance: 0.08 },
//   { feature: 'zipCode', importance: 0.04 },
// ]

LIME: Local Explanations

LIME (Local Interpretable Model-Agnostic Explanations) works by creating small perturbations around a single prediction and fitting a simple, interpretable model to those perturbations. Think of it as zooming into one point on your performance timeline. You don't need to understand the entire app, just what happened in that 342ms window.

SHAP: Fair Credit Assignment

SHAP (SHapley Additive exPlanations) borrows from game theory. Imagine your features are team members on a project. SHAP asks: "If we built every possible subset of this team, how much does each member contribute on average?" This gives each feature a fair share of the prediction.

The key difference: LIME is fast and local, SHAP is thorough and theoretically grounded. Like the difference between a quick console.log debug session and a full profiling run.

When Models Learn the Wrong Thing

A famous example: a model trained to detect pneumonia in X-rays learned to look for the text "PORTABLE" stamped on images, because portable X-ray machines were used for the sickest patients. The model achieved high accuracy by reading labels instead of analyzing lungs.

Without interpretability, you wouldn't know your model is cheating. It's like discovering your "fast" page load is only fast because the service worker is serving stale cached content.

Challenge

Implement a feature importance function that explains which inputs drive a model's predictions.