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Uncertainty in Multiclass Classification

Dr. Rishabh Pathak
1. What is Uncertainty in Classification? Uncertainty refers to the model’s confidence or doubt in its predictions. Quantifying uncertainty is important to understand how reliable each prediction is. In multiclass classification , uncertainty estimates provide probabilities over multiple classes, reflecting how sure the model is about each possible class. 2. Methods to Estimate Uncertainty in Multiclass Classification Most multiclass classifiers provide methods such as: predict_proba: Returns a probability distribution across all classes. decision_function: Returns scores or margins for each class (sometimes called raw or uncalibrated confidence scores). The probability distribution from predict_proba captures the uncertainty by assigning a probability to each class. 3. Shape and Interpretation of predict_proba in Multiclass Output shape: (n_samples, n_classes) Each row corresponds to the probabilities of ...
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Naive Bayes Classifiers

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Dr. Rishabh Pathak

1. What are Naive Bayes Classifiers?

Naive Bayes classifiers are a family of probabilistic classifiers based on applying Bayes' theorem with strong (naive) independence assumptions between the features. Despite their simplicity, they are very effective in many problems, particularly in text classification.

They assume that the features are conditionally independent given the class. This "naive" assumption simplifies computation and makes learning extremely fast.

2. Theoretical Background: Bayes' Theorem

Given an instance x=(x1​,x2​,...,xn​), the predicted class Ck​ is the one that maximizes the posterior probability:

C^=argmaxCk​​P(Ck​∣x)=argmaxCk​​P(x)P(x∣Ck​)P(Ck​)​

Since P(x) is the same for all classes, it can be ignored:

C^=argmaxCk​​P(x∣Ck​)P(Ck​)

The naive assumption factors the likelihood as:

P(x∣Ck​)=∏i=1n​P(xi​∣Ck​)

This reduces the problem of modeling a joint distribution to modeling individual conditional distributions for each feature.

3. Types of Naive Bayes Classifiers in scikit-learn

Three main variants are implemented, each suitable for different types of input data and tasks:

Model

Assumption of Data Type

Application Domain

GaussianNB

Continuous data (Gaussian distribution)

General-purpose use with continuous features; often for high-dimensional datasets.

BernoulliNB

Binary data (presence/absence)

Text classification with binary-valued features (e.g., word occurrence).

MultinomialNB

Discrete count data (e.g., word counts)

Text classification with term frequency or count data (larger documents).
  • GaussianNB assumes data is drawn from Gaussian distributions per class and feature.
  • BernoulliNB models binary features, suitable when features indicate presence or absence.
  • MultinomialNB models feature counts, like word frequencies in text classification.

4. How Naive Bayes Works in Practice

  • During training, Naive Bayes collects simple per-class statistics from each feature independently.
  • It computes estimates of P(xi​∣Ck​) and P(Ck​) from frequency counts or statistics.
  • Because the computations for each feature are independent, training is very fast and scalable.
  • Prediction requires only a simple calculation using these probabilities.

5. Smoothing and the Role of Parameter Alpha

  • To avoid zero probabilities (which would zero out the entire class posterior), the model performs additive smoothing (Laplace smoothing).
  • The parameter α controls the amount of smoothing by adding α "virtual" data points with positive counts to the observed data.
  • Larger α values cause more smoothing and simpler models, which help prevent overfitting.
  • Tuning α is generally not critical but typically improves accuracy.

6. Strengths of Naive Bayes Classifiers

  • Speed: Extremely fast to train and predict; works well on very large datasets.
  • Scalability: Handles high-dimensional sparse data effectively, such as text datasets with thousands or millions of features.
  • Simplicity: Training is straightforward and interpretable.
  • Baseline: Often used as baseline models in classification problems.
  • Performs surprisingly well for many problems despite assuming feature independence.

7. Weaknesses and Limitations

  • The naive independence assumption rarely holds in practice; correlated features can cause suboptimal performance.
  • Generally, less accurate than more sophisticated models like linear classifiers (e.g., Logistic Regression) or ensemble methods.
  • Works only for classification tasks; there are no Naive Bayes models for regression.
  • Not well suited for datasets with complex or non-independent feature relationships.

8. Usage Scenarios

  • Text classification (spam detection, sentiment analysis) where features are word counts or presence indicators.
  • Problems where fast and scalable classification is required, especially with very large, high-dimensional, sparse data.
  • Situations favoring interpretable and simple models for baseline comparisons.

9. Summary

  • Naive Bayes classifiers assign class labels based on Bayesian probability theory with the assumption of feature independence.
  • Three variants accommodate continuous, binary, or count data.
  • They are exceptionally fast and scalable for very large high-dimensional datasets.
  • Generally less accurate than linear models but remain popular for simplicity and speed.
  • Critical parameter smoothing controlled by α usually helps improve performance.

 

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