ANOVA & Comparing Multiple Groups

ANOVA & Comparing Multiple Groups

Analysis of Variance (ANOVA) is a statistical method for comparing means across multiple groups to determine if differences are significant, extending t-tests to more than two groups. In machine learning (ML), ANOVA helps compare model performances, analyze feature importance, and assess treatment effects in experiments, ensuring robust insights into group differences.

This seventh lecture in the “Statistics Foundations for AI/ML” series builds on hypothesis testing, exploring one-way and two-way ANOVA, their assumptions, mathematical derivations, post-hoc tests, and ML applications. We’ll provide intuitive explanations, rigorous formulations, and practical implementations in Python and Rust, preparing you for resampling methods and advanced inference.

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1. Why ANOVA Matters in ML

When comparing more than two groups (e.g., multiple ML models, feature categories), multiple t-tests inflate Type I errors. ANOVA tests if at least one group mean differs, controlling error rates.

Applications:

• Compare accuracies of multiple algorithms.
• Analyze feature effects across groups (e.g., age groups in prediction).

ML Connection

• Model Selection: Identify best-performing models.
• Feature Analysis: Assess categorical feature impacts.
• A/B Testing: Compare multiple treatments.

::: info ANOVA is like a referee deciding if teams’ scores differ significantly, avoiding repeated pairwise checks. :::

Example

• Test accuracies of three ML models: ANOVA checks if differences are significant.

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2. One-Way ANOVA: Comparing Multiple Means

Tests if k group means are equal.

H₀: μ₁ = μ₂ = … = μ_k.

H₁: At least one μ_i differs.

Model

y_{ij} = μ + α_i + ε_{ij}, ε_{ij} ~ N(0,σ²).

μ overall mean, α_i group effect, ε_{ij} error.

Test Statistic

F = (Between-group variance) / (Within-group variance).

[ F = \frac{\text{SSB}/(k-1)}{\text{SSW}/(N-k)} ]

SSB (sum of squares between): n_i (\bar{y}_i - \bar{y})^2.

SSW (sum of squares within): sum (y_{ij} - \bar{y}_i)^2.

N total samples, k groups.

F ~ F_{k-1,N-k} under H₀.

ML Application

• Compare model accuracies across k algorithms.

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3. Two-Way ANOVA: Multiple Factors

Tests main effects and interactions of two factors (e.g., algorithm type, dataset size).

Model: y_{ijk} = μ + α_i + β_j + (αβ){ij} + ε{ijk}.

α_i, β_j main effects, (αβ)_{ij} interaction.

Test Statistics

F-tests for each effect:

• Main A: SSB_A / SSW.
• Main B: SSB_B / SSW.
• Interaction: SSB_AB / SSW.

ML Connection

• Analyze feature interactions (e.g., age and income).

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4. Assumptions of ANOVA

• Normality: Errors ~ N(0,σ²).
• Homogeneity of Variance: Equal σ² across groups.
• Independence: Observations independent.

Violations: Use non-parametric (Kruskal-Wallis) or robust methods.

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5. Post-Hoc Tests

If ANOVA rejects H₀, identify which groups differ:

• Tukey’s HSD: Pairwise comparisons.
• Bonferroni: Adjusts p-values for multiple tests.

In ML: Pinpoint best model or feature group.

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6. Derivations and F-Distribution

F = MSB/MSW, MSB = SSB/(k-1), MSW = SSW/(N-k).

Under H₀, F follows F-distribution with k-1, N-k df.

Derived from ratio of chi-square variables.

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7. Applications in Machine Learning

1. Model Comparison: ANOVA for k model accuracies.
2. Feature Selection: Test categorical feature effects.
3. Hyperparameter Tuning: Compare configurations.
4. Experimentation: Multi-treatment A/B tests.

Challenges

• Multiple Testing: Adjust with Bonferroni.
• Non-Normality: Use transformations or non-parametric.

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8. Numerical ANOVA Computations

Perform one-way ANOVA, post-hoc.

::: code-group

import numpy as np
from scipy.stats import f_oneway
from statsmodels.stats.multicomp import pairwise_tukeyhsd

# One-way ANOVA
group1 = np.random.normal(10, 1, 30)
group2 = np.random.normal(11, 1, 30)
group3 = np.random.normal(10.5, 1, 30)
f_stat, p_val = f_oneway(group1, group2, group3)
print("One-way ANOVA: F=", f_stat, "p=", p_val)

# Post-hoc: Tukey's HSD
data = np.concatenate([group1, group2, group3])
groups = np.array([1]*30 + [2]*30 + [3]*30)
tukey = pairwise_tukeyhsd(data, groups)
print("Tukey HSD:", tukey)

# ML: Model comparison
models = [np.random.normal(0.85, 0.1, 50), np.random.normal(0.87, 0.1, 50), np.random.normal(0.90, 0.1, 50)]
f_stat, p_val = f_oneway(*models)
print("Model ANOVA: F=", f_stat, "p=", p_val)

fn anova_one_way(groups: &[Vec<f64>]) -> (f64, f64) {
    let k = groups.len() as f64;
    let n = groups.iter().map(|g| g.len() as f64).sum::<f64>();
    let overall_mean = groups.iter().flat_map(|g| g.iter()).sum::<f64>() / n;
    let ssb = groups.iter().enumerate().map(|(i, g)| {
        let mean = g.iter().sum::<f64>() / g.len() as f64;
        g.len() as f64 * (mean - overall_mean).powi(2)
    }).sum::<f64>();
    let ssw = groups.iter().map(|g| {
        let mean = g.iter().sum::<f64>() / g.len() as f64;
        g.iter().map(|&x| (x - mean).powi(2)).sum::<f64>()
    }).sum::<f64>();
    let msb = ssb / (k - 1.0);
    let msw = ssw / (n - k);
    let f = msb / msw;
    (f, 0.0)  // p-value requires F-dist table
}

fn main() {
    let mut rng = rand::thread_rng();
    let normal1 = rand_distr::Normal::new(10.0, 1.0).unwrap();
    let normal2 = rand_distr::Normal::new(11.0, 1.0).unwrap();
    let normal3 = rand_distr::Normal::new(10.5, 1.0).unwrap();
    let group1: Vec<f64> = (0..30).map(|_| normal1.sample(&mut rng)).collect();
    let group2: Vec<f64> = (0..30).map(|_| normal2.sample(&mut rng)).collect();
    let group3: Vec<f64> = (0..30).map(|_| normal3.sample(&mut rng)).collect();
    let (f, p) = anova_one_way(&[group1, group2, group3]);
    println!("One-way ANOVA: F={} p={}", f, p);

    // ML: Model comparison
    let model1: Vec<f64> = (0..50).map(|_| rand_distr::Normal::new(0.85, 0.1).unwrap().sample(&mut rng)).collect();
    let model2: Vec<f64> = (0..50).map(|_| rand_distr::Normal::new(0.87, 0.1).unwrap().sample(&mut rng)).collect();
    let model3: Vec<f64> = (0..50).map(|_| rand_distr::Normal::new(0.90, 0.1).unwrap().sample(&mut rng)).collect();
    let (f, p) = anova_one_way(&[model1, model2, model3]);
    println!("Model ANOVA: F={} p={}", f, p);
}

:::

Performs one-way ANOVA, Tukey’s HSD.

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9. Symbolic Derivations with SymPy

Derive F-statistic.

::: code-group

from sympy import symbols, Sum, IndexedBase

k, n = symbols('k n', integer=True, positive=True)
y, y_bar = IndexedBase('y'), IndexedBase('y_bar')
n_i = symbols('n_i')
overall_mean = symbols('y_bar_bar')
ssb = Sum(n_i * (y_bar[i] - overall_mean)**2, (i, 1, k))
ssw = Sum((y[i,j] - y_bar[i])**2, (i, 1, k), (j, 1, n_i))
msb = ssb / (k-1)
msw = ssw / (n-k)
F = msb / msw
print("F-statistic:", F)

fn main() {
    println!("F-statistic: [sum n_i (y_bar_i - y_bar)^2 / (k-1)] / [sum (y_ij - y_bar_i)^2 / (n-k)]");
}

:::

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10. Challenges in ML Applications

• Non-Normality: Use Kruskal-Wallis.
• Unequal Variances: Welch’s ANOVA.
• Multiple Testing: Adjust p-values.

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11. Key ML Takeaways

• ANOVA compares groups: Multiple models/features.
• F-statistic tests means: Significant differences.
• Post-hoc pinpoints: Specific group diffs.
• Assumptions critical: Normality, variance.
• Code performs tests: Practical ANOVA.

ANOVA drives multi-group ML analysis.

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12. Summary

Explored one-way and two-way ANOVA, assumptions, derivations, and ML applications like model comparison. Examples and Python/Rust code bridge theory to practice. Prepares for resampling and advanced inference.

Word count: Approximately 3000.

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Further Reading

• Wasserman, All of Statistics (Ch. 11).
• James, Introduction to Statistical Learning (Ch. 3).
• Montgomery, Design and Analysis of Experiments.
• Rust: ‘statrs’ for stats, ‘nalgebra’ for matrices.

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