Typical Supervised Learning Problem Setting
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1 Typical Supervised Learning Problem Setting Given a set (database) of observations Each observation (x1,, xn, y) Xi are input variables Y is a particular output Build a model to predict y = f(x1,, xn) First define criterion to measure model quality Split dataset into training and test sets Build model using training set Validate model using test set A Database (Example) X1 X2 X3 X4 X5 X6 Y f(x1,,x6) O GOOD GOOD O GOOD GOOD O GOOD GOOD O GOOD GOOD O GOOD GOOD O GOOD GOOD O BAD BAD O GOOD GOOD O GOOD GOOD O GOOD BAD O GOOD GOOD O GOOD GOOD O BAD BAD O GOOD GOOD O GOOD GOOD O BAD GOOD O GOOD GOOD O GOOD GOOD O BAD BAD O GOOD GOOD O BAD BAD O GOOD GOOD O GOOD GOOD O GOOD GOOD O GOOD GOOD O GOOD GOOD 1
2 Main Steps Select subset of relevant input variables Build a model using these variables Generate sequence of models Identify one (or various) as being good models Use only the training set Validate selected models Quantitatively : using the test set Qualitatively : using expert knowledge Main Classes of Methods Supervised learning (= input/output models) Decision/regression trees Neural networks Unsupervised learning (=p(x1,,xn) models) Bayesian networks Clustering 2
3 Inductive Learning Learning from examples The general problem of inductive inference Inductive bias Examples Training Examples for Concept Enjoy Sport Concept: days on which my friend Aldo enjoys his favourite water sports Task: predict the value of Enjoy Sport for an arbitrary day based on the values of the other attributes Sky Sunny Sunny Rainy Sunny Temp Warm Warm Cold Warm Humid rmal Wind instance Water Warm Warm Warm Cool Forecast Same Same Change Change Enjoy Sport 3
4 Inductive Learning Hypothesis Any hypothesis found to approximate the target function well over the training examples, will also approximate the target function well over the unobserved examples. Futility of Bias-Free Learning A learner that makes no prior assumptions regarding the identity of the target concept has no rational basis for classifying any unseen instances. Free Lunch! 4
5 Decision Trees Decision tree representation ID3 learning algorithm Entropy, information gain Overfitting What Is a Decision Tree? Value of X1 Small Medium or Large Value of X2 Y is big < 0.34 > 0.34 Y is very big Y is small 5
6 6 Training Examples Mild Rain D14 rmal Hot Overcast D13 Mild Overcast D12 rmal Mild Sunny D11 rmal Mild Rain D10 rmal Cold Sunny D9 Mild Sunny D8 rmal Cool Overcast D7 rmal Cool Rain D6 rmal Cool Rain D5 Mild Rain D4 Hot Overcast D3 Hot Sunny D2 Hot Sunny D1 Play Tennis Wind Humidity Temp. Outlook Day Decision Tree for PlayTennis Outlook Sunny Overcast Rain Humidity rmal Wind
7 Decision Tree for PlayTennis Outlook Sunny Overcast Rain Humidity Each internal node tests an attribute rmal Each branch corresponds to an attribute value node Each leaf node assigns a classification Decision Tree for PlayTennis Outlook Temperature Humidity Wind PlayTennis Sunny Hot? Outlook Sunny Overcast Rain Humidity Wind rmal 7
8 Decision Tree for Conjunction Outlook=Sunny Wind= Outlook Sunny Overcast Rain Wind Decision Tree for Disjunction Outlook=Sunny Wind= Outlook Sunny Overcast Rain Wind Wind 8
9 Decision Tree for XOR Outlook=Sunny XOR Wind= Outlook Sunny Overcast Rain Wind Wind Wind Decision Tree decision trees represent disjunctions of conjunctions Outlook Sunny Overcast Rain Humidity Wind rmal (Outlook=Sunny Humidity=rmal) (Outlook=Overcast) (Outlook=Rain Wind=) 9
10 When to consider Decision Trees Instances describable by attribute-value pairs Target function is discrete valued Disjunctive hypothesis may be required Possibly noisy training data Missing attribute values Examples: Medical diagnosis Credit risk analysis Object classification for robot manipulator (Tan 1993) Growing and Pruning Pictorially Data mis-fit Underfitting Overfitting Pruning Growing Tree complexity Final tree 10
11 An Application in Bioinformatics Genetics of complex traits Data base (see) Composed of observations on 1086 animals Inputs : 20x2 genetic markers Outputs : phenotypic measurements (numbers) Identify the location of involved chromosomal regions Results : unpruned and pruned Another Application in Bioinformatics Identification of protein origin Data base (see) Composed of frequency of aminoacids in different families Inputs : 20 frequencies Outputs : class of protein Objective: identify the family of the protein 11
12 Yet Another Application in Bioinformatics Identification of regulatory mechanisms between yeast genes Data from microarray experiments [Spellman et al., (1998). Comprehensive Identification of Cell Cycle-regulated Genes of the Yeast Saccharomyces cerevisiae by Microarray Hybridization. Molecular Biology of the Cell 9, ] Want to predict which genes activate: CLN1, CLN2, CLN3, SW14 Decision tree for CNL1 activation YPL256C CLN2 <-0,375-0,375 CLN1 Não activo YPL120C CLB5 <-0,285-0,285 CLN1 Não activo YDR328C SKP1 <0,695 0,695 CLN1 CLN1 Activo Não activo % 1 0% Confusion matrix 1 22% 100% 12
13 Decision tree for CLN2 activation CLB5 <0 0 CLN2 Activo CLN3 <-0,455-0,455 CLN2 Não activo CLN2 Activo CDH1 <-0,475-0,475 CLN2 Não activo ,6% 1 20% Confusion matrix 1 33,3% 80% Decision tree for CLN3 activation YGL003 CDH1 < CLN3 Não activo SKP1 < CLN3 CDC53 Não activo <0.025 >0,025 CLN3 CLN3 Não activo Activo ,3% 16,6% 1 14,2% 85,7% Confusion matrix 13
14 Decision tree for SW14 activation MBP1 < <-0,28 SW14 Não activo MCM1-0,28 CLB1 SIC1 CLN2 < SW14 Activo % 9% 1 37% 90% <1,24 >1,24 SW14 Activo SW14 Não activo <0.025 >0,025 SW14 Não activo SW14 Activo Confusion matrix Top-Down Induction of Decision Trees ID3 1. A the best decision attribute for next node 2. Assign A as decision attribute for node 3. For each value of A create new descendant 4. Sort training examples to leaf node according to the attribute value of the branch 5. If all training examples are perfectly classified (same value of target attribute) stop, else iterate over new leaf nodes. 14
15 Which Attribute is best? [29+,35-] A 1 =? A 2 =? [29+,35-] True False True False [21+, 5-] [8+, 30-] [18+, 33-] [11+, 2-] Entropy S is a sample of training examples p + is the proportion of positive examples p - is the proportion of negative examples Entropy measures the impurity of S Entropy(S) = -p + log 2 p + -p - log 2 p - 15
16 Entropy Entropy(S)= expected number of bits needed to encode class (+ or -) of randomly drawn members of S (under the optimal, shortest length-code) Why? Information theory optimal length code assign log 2 p bits to messages having probability p. So the expected number of bits to encode (+ or -) of random member of S: -p + log 2 p + -p - log 2 p - Information Gain Gain(S,A): expected reduction in entropy due to sorting S on attribute A Gain(S,A)=Entropy(S) - v values(a) S v / S Entropy(S v ) Entropy([29+,35-]) = -29/64 log 2 29/64 35/64 log 2 35/64 = 0.99 [29+,35-] A 1 =? A 2 =? [29+,35-] True False True False [21+, 5-] [8+, 30-] [18+, 33-] [11+, 2-] 16
17 Entropy([21+,5-]) = 0.71 Entropy([8+,30-]) = 0.74 Gain(S,A 1 )=Entropy(S) -26/64*Entropy([21+,5-]) -38/64*Entropy([8+,30-]) =0.27 Information Gain Entropy([18+,33-]) = 0.94 Entropy([8+,30-]) = 0.62 Gain(S,A 2 )=Entropy(S) -51/64*Entropy([18+,33-]) -13/64*Entropy([11+,2-]) =0.12 [29+,35-] A 1 =? A 2 =? [29+,35-] True False True False [21+, 5-] [8+, 30-] [18+, 33-] [11+, 2-] Day D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 D11 D12 D13 D14 Training Examples Outlook Temp. Humidity Wind Sunny Hot Sunny Hot Overcast Hot Rain Mild Rain Cool rmal Rain Cool rmal Overcast Cool rmal Sunny Mild Sunny Cold rmal Rain Mild rmal Sunny Mild rmal Overcast Mild Overcast Hot rmal Rain Mild Play Tennis 17
18 Selecting the Next Attribute S=[9+,5-] E=0.940 Humidity S=[9+,5-] E=0.940 Wind rmal [3+, 4-] [6+, 1-] E=0.985 Gain(S,Humidity) =0.940-(7/14)*0.985 (7/14)*0.592 =0.151 E=0.592 [6+, 2-] [3+, 3-] E=0.811 E=1.0 Gain(S,Wind) =0.940-(8/14)*0.811 (6/14)*1.0 =0.048 Selecting the Next Attribute Sunny S=[9+,5-] E=0.940 Outlook Over cast Rain [2+, 3-] [4+, 0] [3+, 2-] E=0.971 E=0.0 E=0.971 Gain(S,Outlook) =0.940-(5/14)* (4/14)*0.0 (5/14)* =
19 ID3 Algorithm [D1,D2,,D14] [9+,5-] Outlook Sunny Overcast Rain S sunny =[D1,D2,D8,D9,D11] [2+,3-] [D3,D7,D12,D13] [4+,0-] [D4,D5,D6,D10,D14] [3+,2-]?? Gain(S sunny, Humidity)=0.970-(3/5)0.0 2/5(0.0) = Gain(S sunny, Temp.)=0.970-(2/5)0.0 2/5(1.0)-(1/5)0.0 = Gain(S sunny, Wind)=0.970= -(2/5)1.0 3/5(0.918) = ID3 Algorithm Outlook Sunny Overcast Rain Humidity [D3,D7,D12,D13] Wind rmal [D1,D2] [D8,D9,D11] [D6,D14] [D4,D5,D10] 19
20 Hypothesis Space Search ID3 A A A2 A A A4 Hypothesis Space Search ID3 Hypothesis space is complete! Target function surely in there Outputs a single hypothesis backtracking on selected attributes (greedy search) Local minimal (suboptimal splits) Statistically-based search choices Robust to noisy data Inductive bias (search bias) Prefer shorter trees over longer ones Place high information gain attributes close to the root 20
21 Inductive Bias in ID3 H is the power set of instances X Unbiased? Preference for short trees, and for those with high information gain attributes near the root Bias is a preference for some hypotheses, rather than a restriction of the hypothesis space H Occam s razor: prefer the shortest (simplest) hypothesis that fits the data Occam s Razor Why prefer short hypotheses? Argument in favor: Fewer short hypotheses than long hypotheses A short hypothesis that fits the data is unlikely to be a coincidence A long hypothesis that fits the data might be a coincidence Argument opposed: There are many ways to define small sets of hypotheses E.g. All trees with a prime number of nodes that use attributes beginning with Z What is so special about small sets based on size of hypothesis 21
22 Overfitting in Decision Tree Learning Avoid Overfitting How can we avoid overfitting? Stop growing when data split not statistically significant Grow full tree then post-prune Minimum description length (MDL): Minimize: size(tree) + size(misclassifications(tree)) 22
23 Reduced-Error Pruning Split data into training and validation set Do until further pruning is harmful: 1. Evaluate impact on validation set of pruning each possible node (plus those below it) 2. Greedily remove the one that most improves the validation set accuracy Produces smallest version of most accurate subtree Effect of Reduced Error Pruning 23
24 Rule-Post Pruning 1. Convert tree to equivalent set of rules 2. Prune each rule independently of each other 3. Sort final rules into a desired sequence to use Method used in C4.5 Converting a Tree to Rules Outlook Sunny Overcast Rain Humidity Wind rmal R 1 : If (Outlook=Sunny) (Humidity=) Then PlayTennis= R 2 : If (Outlook=Sunny) (Humidity=rmal) Then PlayTennis= R 3 : If (Outlook=Overcast) Then PlayTennis= R 4 : If (Outlook=Rain) (Wind=) Then PlayTennis= R 5 : If (Outlook=Rain) (Wind=) Then PlayTennis= 24
25 Continuous Valued Attributes Create a discrete attribute to test continuous Temperature = C (Temperature > C) = {true, false} Where to set the threshold? Temperatur 15 0 C 18 0 C 19 0 C 22 0 C 24 0 C 27 0 C PlayTennis (see paper by [Fayyad, Irani 1993] Attributes with many Values Problem: if an attribute has many values, maximizing InformationGain will select it. E.g.: Imagine using Date= as attribute perfectly splits the data into subsets of size 1 Use GainRatio instead of information gain as criteria: GainRatio(S,A) = Gain(S,A) / SplitInformation(S,A) SplitInformation(S,A) = -Σ i=1..c S i / S log 2 S i / S Where S i is the subset for which attribute A has the value v i 25
26 Attributes with Cost Consider: Medical diagnosis : blood test costs 1000 SEK Robotics: width_from_one_feet has cost 23 secs. How to learn a consistent tree with low expected cost? Replace Gain by : Gain 2 (S,A)/Cost(A) [Tan, Schimmer 1990] 2 Gain(S,A) -1/(Cost(A)+1) w w [0,1] [Nunez 1988] Unknown Attribute Values What is some examples missing values of A? Use training example anyway sort through tree If node n tests A, assign most common value of A among other examples sorted to node n. Assign most common value of A among other examples with same target value Assign probability pi to each possible value vi of A Assign fraction pi of example to each descendant in tree Classify new examples in the same fashion 26
27 Cross-Validation Estimate the accuracy of a hypothesis induced by a supervised learning algorithm Predict the accuracy of a hypothesis over future unseen instances Select the optimal hypothesis from a given set of alternative hypotheses Pruning decision trees Model selection Feature selection Combining multiple classifiers (boosting) Holdout Method Partition data set D = {(v 1,y 1 ),,(v n,y n )} into training D t and validation set D h =D\D t Training D t Validation D\D t acc h = 1/h (vi,yi) Dh δ(i(d t,v i ),y i ) I(D t,v i ) : output of hypothesis induced by learner I trained on data D t for instance v i δ(i,j) = 1 if i=j and 0 otherwise Problems: makes insufficient use of data training and validation set are correlated 27
28 Cross-Validation k-fold cross-validation splits the data set D into k mutually exclusive subsets D 1,D 2,,D k D 1 D 2 D 3 D 4 Train and test the learning algorithm k times, each time it is trained on D\D i and tested on D i D 1 D 2 D 3 D 4 D 1 D 2 D 3 D 4 D 1 D 2 D 3 D 4 D 1 D 2 D 3 D 4 acc cv = 1/n (vi,yi) D δ(i(d\d i,v i ),y i ) Cross-Validation Uses all the data for training and testing Complete k-fold cross-validation splits the dataset of size m in all (m over m/k) possible ways (choosing m/k instances out of m) Leave n-out cross-validation sets n instances aside for testing and uses the remaining ones for training (leave one-out is equivalent to n-fold crossvalidation) In stratified cross-validation, the folds are stratified so that they contain approximately the same proportion of labels as the original data set 28
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