1. On Discriminative vs. Generative classifiers: Naïve Bayes
Presenter : Seung Hwan, Bae

2. Andrew Y. Ng and Michael I. Jordan
Neural Information Processing System (NIPS), 2001
(slides adapted from Ke Chen from University of Manchester and YangQiu Song from MSRA)
Total Citation: 831

3. Machine Learning

4. Generative vs. Discriminative Classifiers
Training classifiers involves estimating f: X->Y, or P(Y|X)
X: Training data, Y: Labels
Discriminative classifiers(also called ‘informative’ by Rubinstein & Hastie):
Assume some functional form from for P(Y|X)
Estimate parameters of P(Y|X) directly from training data
Generative classifier
Assume some functional from for P(X|Y), P(X)
Estimate parameters of P(X|Y), P(X) directly from training data
Use Bayes rule to calculate 𝑃 𝑌|𝑋= 𝑥 𝑖

5. Bayes Formula

6. Generative Model
Color
Size
Texture
Weight …

7. Discriminative Model
Logistic Regression
Color
Size
Texture
Weight …

8. Comparison Generative models Discriminative models
Assume some functional form for P(X|Y), P(Y)
Estimate parameters of P(X|Y), P(Y) directly from training data
Use Bayes rule to calculate P(Y|X=x)
Discriminative models
Directly assume some functional form for P(Y|X)
Estimate parameters of P(Y|X) directly from training data Y Y X1 X1 X2 X2
Naïve Bayes
Generative
Logistic Regression
Discriminative

9. Probability Basics
Prior, conditional and joint probability for random variables
Prior probability:
Conditional probability:
Joint probability:
Relationship:
Independence:
Bayesian Rule

10. Probabilistic Classification
Establishing a probabilistic model for classification
Discriminative model
Discriminative
Probabilistic Classifier

11. Probabilistic Classification
Establishing a probabilistic model for classification (cont.)
Generative model
Generative
Probabilistic Model
for Class 1
for Class 2
for Class L

12. Probabilistic Classification
MAP classification rule
MAP: Maximum A Posterior
Assign x to c* if
Generative classification with the MAP rule
Apply Bayesian rule to convert them into posterior probabilities
Then apply the MAP rule

13. Naïve Bayes Bayes classification
Difficulty: learning the joint probability
If the number of feature n is large or when a feature can take on a large number of values, then basing such a model on probability tables is infeasible.

14. Naïve Bayes Naïve Bayes classification
Assume that all input attributes are conditionally independent!
MAP classification rule: for

15. Naïve Bayes Naïve Bayes Algorithm (for discrete input attributes)
Learning phase: Given a train set S,
Output: conditional probability tables; for elements
Test phase: Given an unknown instance
Look up tables to assign the label c* to X’ if

16. Example
Example: Play Tennis

17. Example Learning phase 2/9 3/5 4/9 0/5 3/9 2/5 2/9 2/5 4/9 3/9 1/5 3/9
Outlook
Play=Yes
Play=No
Sunny
2/9
3/5
Overcast
4/9
0/5
Rain
3/9
2/5
Temperature
Play=Yes
Play=No
Hot
2/9
2/5
Mild
4/9
Cool
3/9
1/5
Humidity
Play=Yes
Play=No
High
3/9
4/5
Normal
6/9
1/5
Wind
Play=Yes
Play=No
Strong
3/9
3/5
Weak
6/9
2/5
P(Play=Yes) = 9/14
P(Play=No) = 5/14

18. Example Test Phase Given a new instances Look up tables MAP rule
x’=(Outlook=Sunny, Temperature=Cool, Humidity=High, Wind=Strong)
Look up tables
MAP rule
P(Outlook=Sunny|Play=Yes) = 2/9
P(Temperature=Cool|Play=Yes) = 3/9
P(Huminity=High|Play=Yes) = 3/9
P(Wind=Strong|Play=Yes) = 3/9
P(Play=Yes) = 9/14
P(Outlook=Sunny|Play=No) = 3/5
P(Temperature=Cool|Play==No) = 1/5
P(Huminity=High|Play=No) = 4/5
P(Wind=Strong|Play=No) = 3/5
P(Play=No) = 5/14
P(Yes|x’): [P(Sunny|Yes)P(Cool|Yes)P(High|Yes)P(Strong|Yes)]P(Play=Yes) =
P(No|x’): [P(Sunny|No) P(Cool|No)P(High|No)P(Strong|No)]P(Play=No) =
Given the fact P(Yes|x’) < P(No|x’), we label x’ to be “No”.

19. Example Test Phase Given a new instance,
x’=(Outlook=Sunny, Temperature=Cool, Humidity=High, Wind=Strong)
Look up tables
MAP rule
P(Outlook=Sunny|Play=Yes) = 2/9
P(Temperature=Cool|Play=Yes) = 3/9
P(Huminity=High|Play=Yes) = 3/9
P(Wind=Strong|Play=Yes) = 3/9
P(Play=Yes) = 9/14
P(Outlook=Sunny|Play=No) = 3/5
P(Temperature=Cool|Play==No) = 1/5
P(Huminity=High|Play=No) = 4/5
P(Wind=Strong|Play=No) = 3/5
P(Play=No) = 5/14
P(Yes|x’): [P(Sunny|Yes)P(Cool|Yes)P(High|Yes)P(Strong|Yes)]P(Play=Yes) =
P(No|x’): [P(Sunny|No) P(Cool|No)P(High|No)P(Strong|No)]P(Play=No) =
Given the fact P(Yes|x’) < P(No|x’), we label x’ to be “No”. 19

20. Relevant Issues Violation of Independent Assumption
For many real world tasks,
Nevertheless, naïve Bayes works surprisingly well anyway!
Zero conditional probability problem
In no example contains the attribute value
In this circumstance, during test
For a remedy, conditional probabilities estimated with

21. Relevant Issues Continuous-valued Input Attributes
Numberless vales for an attribute
Conditional probability modeled with the normal distribution
Learning phase:
Output: normal distributions and
Test phase:
Calculate conditional probabilities with all the normal distribution
Apply the MAP rule to make a decision

22. Advantages of Naïve Bayes
Naïve Bayes based on the independent assumption
A small amount of training data to estimate parameters (means and variances of the variable)
Only the variances of variables for each class need to be determined and not the entire covariance matrix
Test is straightforward; just looking up tables or calculating conditional probabilities with normal distribution

23. Conclusion
Performance competitive to most of state-of-art classifiers even in presence of violating independence assumption
Many successful application, e.g., spam mail fitering
A good candidate of a base learner in ensemble learning
Apart from classification, naïve Bayes can do more…