Decision Trees Lecture 12
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1 Decision Trees Lecture 12 David Sontag New York University Slides adapted from Luke Zettlemoyer, Carlos Guestrin, and Andrew Moore
2 Machine Learning in the ER Physician documentation Triage Information (Free text) MD comments (free text) Specialist consults 2 hrs 30 min T=0 Lab results (Continuous valued) Repeated vital signs (continuous values) Measured every 30 s Disposition
3 Can we predict infec>on? Triage Information (Free text) MD comments (free text) Specialist consults Physician documentation Many crucial decisions about a patient s care are made here! Lab results (Continuous valued) Repeated vital signs (continuous values) Measured every 30 s
4 Can we predict infec>on? Previous automa>c approaches based on simple criteria: Temperature < 96.8 F or > F Heart rate > 90 beats/min Respiratory rate > 20 breaths/min Too simplified e.g., heart rate depends on age!
5 Can we predict infec>on? These are the ayributes we have for each pa>ent: Temperature Heart rate (HR) Respiratory rate (RR) Age Acuity and pain level Diastolic and systolic blood pressure (DBP, SBP) Oxygen Satura>on (SaO2) We have these ayributes + label (infec>on) for 200,000 pa>ents! Let s learn to classify infec>on
6 Predic>ng infec>on using decision trees
7 Hypotheses: decision trees f : X Y Each internal node tests an attribute x i Each branch assigns an attribute value x i =v Each leaf assigns a class y To classify input x: traverse the tree from root to leaf, output the labeled y Cylinders good bad bad Maker america asia europe bad good good bad Horsepower low med high good bad Human interpretable!
8 How many possible hypotheses? What functions can be represented? Hypothesis space Cylinders good bad bad Maker Horsepower america asia europe low med high bad good good bad good bad
9 What func>ons can be represented? Decision trees can represent any func>on of the input ayributes! A B A xor B F F F F T T T F T T T F F F B A F T B T F T T T F For Boolean func>ons, path to leaf gives truth table row Cylinders (Figure from Stuart Russell) But, could require exponen>ally many nodes good bad bad Maker Horsepower america asia europe low med high bad good good bad good bad cyl=3 (cyl=4 (maker=asia maker=europe))
10 Are all decision trees equal? Many trees can represent the same concept But, not all trees will have the same size! e.g., φ = (A B) ( A C) - - ((A and B) or (not A and C)) A B C t t f f + _ t f + _ Which tree do we prefer? B C C t f f + t f + _ A t f A _ + _ t t f
11 Learning decision trees is hard!!! Learning the simplest (smallest) decision tree is an NP- complete problem [Hyafil & Rivest 76] Resort to a greedy heuris>c: Start from empty decision tree Split on next best a4ribute (feature) Recurse
12 A Decision Stump
13 Key idea: Greedily learn trees using recursion Records in which cylinders = 4 Take the Original Dataset.. And partition it according to the value of the attribute we split on Records in which cylinders = 5 Records in which cylinders = 6 Records in which cylinders = 8
14 Recursive Step Build tree from These records.. Build tree from These records.. Build tree from These records.. Build tree from These records.. Records in which cylinders = 4 Records in which cylinders = 5 Records in which cylinders = 6 Records in which cylinders = 8
15 Second level of tree Recursively build a tree from the seven records in which there are four cylinders and the maker was based in Asia (Similar recursion in the other cases)
16 A full tree
17 Spliong: choosing a good ayribute Would we prefer to split on X 1 or X 2? t Y=t : 4 Y=f : 0 X 1 f Y=t : 1 Y=f : 3 t Y=t : 3 Y=f : 1 X 2 f Y=t : 2 Y=f : 2 Idea: use counts at leaves to define probability distributions, so we can measure uncertainty! X 1 X 2 Y T T T T F T T T T T F T F T T F F F F T F F F F
18 Measuring uncertainty Good split if we are more certain about classifica>on aper split Determinis>c good (all true or all false) Uniform distribu>on bad What about distribu>ons in between? P(Y=A) = 1/2 P(Y=B) = 1/4 P(Y=C) = 1/8 P(Y=D) = 1/8 P(Y=A) = 1/4 P(Y=B) = 1/4 P(Y=C) = 1/4 P(Y=D) = 1/4
19 Entropy Entropy H(Y) of a random variable Y Entropy of a coin flip More uncertainty, more entropy! Information Theory interpretation: H(Y) is the expected number of bits needed to encode a randomly drawn value of Y (under most efficient code) Entropy Probability of heads
20 High Entropy High, Low Entropy Y is from a uniform like distribu>on Flat histogram Values sampled from it are less predictable Low Entropy Y is from a varied (peaks and valleys) distribu>on Histogram has many lows and highs Values sampled from it are more predictable (Slide from Vibhav Gogate)
21 Entropy Example Entropy of a coin flip Entropy Probability of heads P(Y=t) = 5/6 P(Y=f) = 1/6 H(Y) = - 5/6 log 2 5/6-1/6 log 2 1/6 = 0.65 X 1 X 2 Y T T T T F T T T T T F T F T T F F F
22 Condi>onal Entropy Condi>onal Entropy H( Y X) of a random variable Y condi>oned on a random variable X Example: P(X 1 =t) = 4/6 P(X 1 =f) = 2/6 t Y=t : 4 Y=f : 0 X 1 f Y=t : 1 Y=f : 1 H(Y X 1 ) = - 4/6 (1 log log 2 0) - 2/6 (1/2 log 2 1/2 + 1/2 log 2 1/2) = 2/6 X 1 X 2 Y T T T T F T T T T T F T F T T F F F
23 Informa>on gain Decrease in entropy (uncertainty) aper spliong In our running example: IG(X 1 ) = H(Y) H(Y X 1 ) = IG(X 1 ) > 0 we prefer the split! X 1 X 2 Y T T T T F T T T T T F T F T T F F F
24 Learning decision trees Start from empty decision tree Split on next best a4ribute (feature) Use, for example, informa>on gain to select ayribute: Recurse
25 When to stop? First split looks good! But, when do we stop?
26 Base Case One Don t split a node if all matching records have the same output value
27 Base Case Two Don t split a node if data points are identical on remaining attributes
28 Base Cases: An idea Base Case One: If all records in current data subset have the same output then don t recurse Base Case Two: If all records have exactly the same set of input ayributes then don t recurse Proposed Base Case 3: If all attributes have small information gain then don t recurse This is not a good idea
29 The problem with proposed case 3 y = a XOR b The information gains:
30 If we omit proposed case 3: y = a XOR b The resulting decision tree: Instead, perform pruning after building a tree
31 Decision trees will overfit
32 Decision trees will overfit Standard decision trees have no learning bias Training set error is always zero! (If there is no label noise) Lots of variance Must introduce some bias towards simpler trees Many strategies for picking simpler trees Fixed depth Fixed number of leaves Random forests
33 Real- Valued inputs What should we do if some of the inputs are real- valued? Infinite number of possible split values!!!
34 One branch for each numeric value idea: Hopeless: hypothesis with such a high branching factor will shatter any dataset and overfit
35 Threshold splits Binary tree: split on ayribute X at value t One branch: X < t Other branch: X t Requires small change Allow repeated splits on same variable along a path bad Year <78 78 Year bad good <70 70 good
36 The set of possible thresholds Binary tree, split on ayribute X One branch: X < t Other branch: X t Search through possible values of t Seems hard!!! But only a finite number of t s are important: X j tc c 1 t 2 Sort data according to X into {x 1,,x m } Consider split points of the form x i + (x i+1 x i )/2 Morever, only splits between examples of different classes mayer! X j tc c 1 t 2 (Figures from Stuart Russell)
37 Picking the best threshold Suppose X is real valued with threshold t Want IG(Y X:t), the informa>on gain for Y when tes>ng if X is greater than or less than t Define: H(Y X:t) = p(x < t) H(Y X < t) + p(x >= t) H(Y X >= t) IG(Y X:t) = H(Y) - H(Y X:t) IG*(Y X) = max t IG(Y X:t) Use: IG*(Y X) for con>nuous variables
38 Example with MPG
39 Example tree for our con>nuous dataset
40 What you need to know about decision trees Decision trees are one of the most popular ML tools Easy to understand, implement, and use Computa>onally cheap (to solve heuris>cally) Informa>on gain to select ayributes (ID3, C4.5, ) Presented for classifica>on, can be used for regression and density es>ma>on too Decision trees will overfit!!! Must use tricks to find simple trees, e.g., Fixed depth/early stopping Pruning Or, use ensembles of different trees (random forests)
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