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TRANSCRIPT
INTRODUCTION
TO
MACHINE
LEARNING 3RD EDITION
ETHEM ALPAYDIN
© The MIT Press, 2014
http://www.cmpe.boun.edu.tr/~ethem/i2ml3e
Lecture Slides for
CHAPTER 19: DESIGN AND ANALYSIS OF MACHINE LEARNING EXPERIMENTS
Introduction 3
Questions:
Assessment of the expected error of a learning algorithm: Is
the error rate of 1-NN less than 2%?
Comparing the expected errors of two algorithms: Is k-NN
more accurate than MLP ?
Training/validation/test sets
Resampling methods: K-fold cross-validation
Algorithm Preference 4
Criteria (Application-dependent):
Misclassification error, or risk (loss functions)
Training time/space complexity
Testing time/space complexity
Interpretability
Easy programmability
Cost-sensitive learning
Factors and Response
Response function based on output to be maximized
Depends on controllable factors
Uncontrollable factors introduce randomness
Find the configuration of controllable factors that maximizes response and minimally affected by uncontrollable factors
5
Strategies of Experimentation 6
Response surface design for approximating and maximizing
the response function in terms of the controllable factors
How to search the factor space?
Guidelines for ML experiments 7
A. Aim of the study
B. Selection of the response variable
C. Choice of factors and levels
D. Choice of experimental design
E. Performing the experiment
F. Statistical Analysis of the Data
G. Conclusions and Recommendations
The need for multiple training/validation sets
{Xi,Vi}i: Training/validation sets of fold i
K-fold cross-validation: Divide X into k, Xi,i=1,...,K
Ti share K-2 parts
Resampling and
K-Fold Cross-Validation 8
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Bootstrapping 10
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Draw instances from a dataset with replacement
Prob that we do not pick an instance after N draws
that is, only 36.8% is new!
Performance Measures 11
Error rate = # of errors / # of instances = (FN+FP) / N
Recall = # of found positives / # of positives
= TP / (TP+FN) = sensitivity = hit rate
Precision = # of found positives / # of found
= TP / (TP+FP)
Specificity = TN / (TN+FP)
False alarm rate = FP / (FP+TN) = 1 - Specificity
ROC Curve 12
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Precision and Recall 14
Interval Estimation 15
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When σ2 is not known:
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Reject a null hypothesis if not supported by the sample with enough confidence
X = { xt }t where xt ~ N ( μ, σ2)
H0: μ = μ0 vs. H1: μ ≠ μ0
Accept H0 with level of significance α if μ0 is in the
100(1- α) confidence interval
Two-sided test
Hypothesis Testing 17
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One-sided test: H0: μ ≤ μ0 vs. H1: μ > μ0
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Assessing Error: H0:p ≤ p0 vs. H1:p > p0
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Single training/validation set: Binomial Test
If error prob is p0, prob that there are e errors or
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N=100, e=20
Normal Approximation to the Binomial
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Multiple training/validation sets
xti = 1 if instance t misclassified on fold i
Error rate of fold i:
With m and s2 average and var of pi , we accept p0 or less error if
is less than tα,K-1
Paired t Test 21
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Single training/validation set: McNemar’s Test
Under H0, we expect e01= e10=(e01+ e10)/2
Comparing Classifiers: H0:μ0=μ1 vs.
H1:μ0≠μ1 22
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K-Fold CV Paired t Test 23
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pi = pi1 – pi
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The null hypothesis is whether pi has mean 0
5×2 cv Paired t Test 24
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pi(j) : difference btw errors of 1 and 2 on fold j=1,
2 of replication i=1,...,5
Two-sided test: Accept H0: μ0 = μ1 if in (-tα/2,5,tα/2,5)
One-sided test: Accept H0: μ0 ≤ μ1 if < tα,5
5×2 cv Paired F Test 25
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Comparing L>2 Algorithms:
Analysis of Variance (Anova) 26
LH 210 :
Errors of L algorithms on K folds
We construct two estimators to σ2 .
One is valid if H0 is true, the other is always valid.
We reject H0 if the two estimators disagree.
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ANOVA table 29
If ANOVA rejects, we do pairwise posthoc tests
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Comparison over Multiple Datasets 30
Comparing two algorithms:
Sign test: Count how many times A beats B over N datasets, and check if this could have been by chance if A and B did have the same error rate
Comparing multiple algorithms
Kruskal-Wallis test: Calculate the average rank of all algorithms on N datasets, and check if these could have been by chance if they all had equal error
If KW rejects, we do pairwise posthoc tests to find which ones have significant rank difference
Instead of testing using a single performance
measure, e.g., error, use multiple measures for
better discrimination, e.g., [fp-rate,fn-rate]
Compare p-dimensional distributions
Parametric case: Assume p-variate Gaussians
Multivariate Tests 31
Multivariate Pairwise Comparison 32
Paired differences:
Hotelling’s multivariate T2 test
For p=1, reduces to paired t test
Multivariate ANOVA 33
Comparsion of L>2 algorithms