AI Bias — Detection, Measurement, and Mitigation

import numpy as np
import pandas as pd
from sklearn.linear_model import LogisticRegression
from sklearn.metrics import accuracy_score

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# TYPES OF BIAS in AI systems
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bias_types = {
    "Historical bias": "Training data reflects past discrimination (e.g., hiring data biased toward men)",
    "Representation bias": "Underrepresentation of minorities in training data -> poor performance on them",
    "Measurement bias": "Proxy variables introduce proxy discrimination (zip code as proxy for race)",
    "Deployment bias": "Model used in context different from training (e.g., tool trained on adults used on children)",
    "Automation bias": "Humans over-rely on AI predictions, amplifying model errors for protected groups",
    "Feedback loops": "Biased predictions -> biased data -> retrained biased model (amplifying bias over time)",
}

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# MEASURING FAIRNESS -- multiple metrics (choose based on context!)
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np.random.seed(42)
n = 2000
protected_attr = np.random.choice([0, 1], n, p=[0.6, 0.4])  # 0=majority, 1=minority
features = np.random.randn(n, 5)
# Inject bias: minority group's true outcome slightly underrepresented in labels
true_labels  = ((features[:, 0] + features[:, 1] + protected_attr * 0.5) > 0).astype(int)
# Simulate biased predictions (model performs worse on minority)
predictions = ((features[:, 0] + features[:, 1] + protected_attr * -0.5) > 0.1).astype(int)

def fairness_metrics(y_true: np.ndarray, y_pred: np.ndarray, protected: np.ndarray) -> dict:
    '''Compute multiple fairness metrics for a binary classifier.'''
    majority_mask = protected == 0
    minority_mask = protected == 1

    # Accuracy per group
    acc_maj = accuracy_score(y_true[majority_mask], y_pred[majority_mask])
    acc_min = accuracy_score(y_true[minority_mask], y_pred[minority_mask])

    # Positive prediction rates per group (for Demographic Parity)
    ppr_maj = y_pred[majority_mask].mean()
    ppr_min = y_pred[minority_mask].mean()

    # True Positive Rates per group (Equal Opportunity)
    tp_maj = ((y_true == 1) & (y_pred == 1) & majority_mask).sum()
    fn_maj = ((y_true == 1) & (y_pred == 0) & majority_mask).sum()
    tpr_maj = tp_maj / (tp_maj + fn_maj + 1e-10)

    tp_min = ((y_true == 1) & (y_pred == 1) & minority_mask).sum()
    fn_min = ((y_true == 1) & (y_pred == 0) & minority_mask).sum()
    tpr_min = tp_min / (tp_min + fn_min + 1e-10)

    # Disparate Impact ratio: minority positive rate / majority positive rate
    # Fair: > 0.8 (US legal standard "80% rule")
    disparate_impact = ppr_min / (ppr_maj + 1e-10)

    return {
        "Accuracy (majority)":      round(acc_maj, 3),
        "Accuracy (minority)":      round(acc_min, 3),
        "Accuracy gap":             round(acc_maj - acc_min, 3),
        "Positive pred. rate (majority)": round(ppr_maj, 3),
        "Positive pred. rate (minority)": round(ppr_min, 3),
        "Disparate Impact":         round(disparate_impact, 3),  # < 0.8 = legally biased!
        "TPR (majority)":           round(tpr_maj, 3),
        "TPR (minority)":           round(tpr_min, 3),
        "Equal Opportunity gap":    round(tpr_maj - tpr_min, 3),
    }

metrics_biased = fairness_metrics(true_labels, predictions, protected_attr)
print("\nFairness Metrics (biased model):")
for metric, value in metrics_biased.items():
    flag = " <-- FAIR" if "gap" not in metric.lower() and 0.8 <= value <= 1.25 else ""
    flag = " <-- BIASED!" if metric == "Disparate Impact" and value < 0.8 else flag
    print(f"  {metric:35s}: {value}{flag}")

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# MITIGATION -- reweighting (pre-processing)
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# Upweight minority samples in training loss to compensate for underrepresentation

sample_weights = np.ones(n)
# Give minority group 2x weight
sample_weights[protected_attr == 1] = 2.0
sample_weights = sample_weights / sample_weights.mean()  # normalize

model = LogisticRegression()
model.fit(features, true_labels, sample_weight=sample_weights)
predictions_fair = model.predict(features)

metrics_fair = fairness_metrics(true_labels, predictions_fair, protected_attr)
print("\nFairness Metrics (re-weighted model):")
for metric, value in metrics_fair.items():
    print(f"  {metric:35s}: {value}")