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Data Mining – Algorithms: Decision Trees - ID3. Chapter 4, Section 4.3. Common Recursive Decision Tree Induction Pseudo Code. If all instances have same classification Stop, create leaf with that classification Else Choose attribute for root

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## Data Mining – Algorithms: Decision Trees - ID3

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**Data Mining – Algorithms: Decision Trees - ID3**Chapter 4, Section 4.3**Common Recursive Decision Tree Induction Pseudo Code**• If all instances have same classification • Stop, create leaf with that classification • Else • Choose attribute for root • Make one branch for each possible value (or range if numeric attribute) • For each branch • Recursively call same method**Choosing an Attribute**• Key to how this works • Main difference between many decision tree learning algorithms • First, let’s look at intuition • We want a small tree • Therefore, we prefer attributes that divide instances well • A perfect attribute to use is one for which each value is associated with only one class (e.g. if all rainy days play=no and all sunny days play=yes and all overcast days play=yes) • We don’t often get perfect very high in the tree • So, we need a way to measure relative purity**Relative Purity – My Weather**• Intuitively, it looks like outlook creates more purity, but computers can’t use intuition • ID3 (famous early algorithm) – measured based on “information theory” (Shannon 1948)**Information Theory**• A piece of information is more valuable if it adds to what is already known • If all instances in a group were known to be all in the same class, then the information value of being told the class of a particular instance is Zero • If instances are evenly split between classes, then the information value of being told the class of a particular instance is Maximized • Hence more purity = less information • Information theory measures the value of information using “entropy”, which is measured in “bits”**Entropy**• Derivation of formula is beyond our scope • Calculation – entropy = (-cnt1 log2cnt1 – cnt2 log2cnt2 – cnt3 log2cnt3 … + totalcnts log2totalcnts ) / totalcnts where cnt1, cnt2, … are counts of number of instances in each class and totalcnts is the total of those counts <see entropy.xls – show how it goes up with less purity>**Information Gain**• Amount entropy is reduced as a result of dividing • This is the deciding measure for ID3**Outlook**Temp Humid Windy Play? sunny hot high FALSE no sunny hot high TRUE yes overcast hot high FALSE no rainy mild high FALSE no rainy cool normal FALSE no rainy cool normal TRUE no overcast cool normal TRUE yes sunny mild high FALSE yes sunny cool normal FALSE yes rainy mild normal FALSE no sunny mild normal TRUE yes overcast mild high TRUE yes overcast hot normal FALSE no rainy mild high TRUE no Example: My Weather (Nominal)**Let’s take this a little more realistic than book does**• this will be cross validated • Normally 10-fold is used, but with 14 instances that is a little awkward • For each fold will divide into training and test data • This time through, let’s save the last record as a test**Using Entropy to Divide**• Entropy for all training instances (5 yes, 8 no) = .96 • Entropy for Outlook division = weighted average of Nodes created by division = 5/13 * .72 (entropy [4,1]) + 4/13 * 1 (entropy [2,2]) + 4/13 * 0 (entropy [0,4]) = .585 • Info Gain = .96 - .585 = .375**Using Entropy to Divide**• Entropy for Temperature division = weighted average of Nodes created by division = 4/13 * .81 (entropy [1,3]) + 5/13 * .97 (entropy [3,2]) + 4/13 * 1.0 (entropy [2,2]) = .931 • Info Gain = .96 - .931 = .029**Using Entropy to Divide**• Entropy for Humidity division = weighted average of Nodes created by division = 6/13 * 1 (entropy [3,3]) + 7/13 * .98 (entropy [3,4]) = .992 • Info Gain = .96 - .992 = -0.032**Using Entropy to Divide**• Entropy for Windy division = weighted average of Nodes created by division = 8/13 * .81 (entropy [2,6]) + 5/13 * .72 (entropy [4,1]) = .777 • Info Gain = .96 - .777 = .183 • Biggest Gain is via Outlook**Recursive Tree Building**• On sunny instances, will consider other attributes**Outlook**Temp Humid Windy Play? sunny hot high FALSE no sunny hot high TRUE yes sunny mild high FALSE yes sunny cool normal FALSE yes sunny mild normal TRUE yes Example: My Weather (Nominal)**Using Entropy to Divide**• Entropy for all sunny training instances (4 yes, 1 no) = .72 • Outlook does not have to be considered because it has already been used**Using Entropy to Divide**• Entropy for Temperature division = weighted average of Nodes created by division = 2/5 * 1.0 (entropy [1,1]) + 2/5 * 0.0 (entropy [2,0]) + 1/5 * 0.0 (entropy [1,0]) = .400 • Info Gain = .72 - .4 = .32**Using Entropy to Divide**• Entropy for Humidity division = weighted average of Nodes created by division = 3/5 * .918 (entropy [2,1]) + 2/5 * .0 (entropy [2,0]) = .551 • Info Gain = .72 - .55 = 0.17**Using Entropy to Divide**• Entropy for Windy division = weighted average of Nodes created by division = 3/5 * .918 (entropy [3,2]) + 2/5 * 0 (entropy [2,0]) = .551 • Info Gain = .72 - .55 = .17 • Biggest Gain is via Temperature**Tree So Far**outlook Sunny Rainy Overcast temp 4 no 2 yes, 2 no Cool Hot Mild 1 yes, 1 no 2 yes 1 yes**Outlook**Temp Humid Windy Play? sunny hot high FALSE no sunny hot high TRUE yes Recursive Tree Building • On sunny, hot instances, will consider other attributes**Tree So Far**outlook Sunny Rainy Overcast temp 4 no 2 yes, 2 no Hot Cool Mild windy 1 yes 2 yes True False 1 yes 1 no**Recursive Tree Building**• On overcast instances, will consider other attributes**Outlook**Temp Humid Windy Play? overcast hot high FALSE no overcast cool normal TRUE yes overcast mild high TRUE yes overcast hot normal FALSE no Example: My Weather (Nominal)**Using Entropy to Divide**• Entropy for all overcast training instances (2 yes, 2 no) = 1.0 • Outlook does not have to be considered because it has already been used**Using Entropy to Divide**• Entropy for Temperature division = weighted average of Nodes created by division = 2/4 * 0.0 (entropy [0,2]) + 1/4 * 0.0 (entropy [1,0]) + 1/4 * 0.0 (entropy [1,0]) = .000 • Info Gain = 1.0 - 0.0 = 1.0**Using Entropy to Divide**• Entropy for Humidity division = weighted average of Nodes created by division = 2/4 * 1.0 (entropy [1,1]) + 2/4 * 1.0 (entropy [1,1]) = 1.0 • Info Gain = 1.0 – 1.0 = 0.0**Using Entropy to Divide**• Entropy for Windy division = weighted average of Nodes created by division = 2/4 * 0.0 (entropy [0,2]) + 2/4 * 0.0 (entropy [2,0]) = 0.0 • Info Gain = 1.0 – 0.0 = 1.0 • Biggest Gain is tie between Temperature and Windy**Tree So Far**outlook Sunny Rainy Overcast temp windy 4 no Hot Cool Mild True False windy 1 yes 2 yes 2 yes 2 no True False 1 yes 1 no**Test Instance**• Top of the tree checks outlook • Test instance value = rainy • Branch right • Reach a leaf • Predict “No” (which is correct)**In a 14-fold cross validation, this would continue 13 more**times • Let’s run WEKA on this …**WEKA results – first look near the bottom**=== Stratified cross-validation === === Summary === Correctly Classified Instances 9 64.2857 % Incorrectly Classified Instances 2 14.2857 % 3 Unclassified Instances ============================================ • On the cross validation – it got 9 out of 14 tests correct (the unclassified instances are tough to understand without seeing all of the trees that were built. It surprises me. They may do some work to avoid overfitting**More Detailed Results**=== Confusion Matrix === a b <-- classified as 2 1 | a = yes 1 7 | b = no ==================================== • Here we see –the program 3 times predicted play=yes, on 2 of those it was correct • The program 8 times predicted play = no, on 7 of those it was correct • There were 3 instances whose actual value was play=yes, the program correctly predicted that on 2 of them • There were 8 instances whose actual value was play=no, the program correctly predicted that on 7 of them • All of the unclassified instances were actually play=yes**Part of our purpose is to have a take-home message for**humans • Not 14 take home messages! • So instead of reporting each of the things learned on each of the 14 training sets … • … The program runs again on all of the data and builds a pattern for that – a take home message**WEKA - Take-Home**• This is a decision tree!! Can you see it? • This is almost the same as we generated, we took a tie breaker a different way • This is a fairly simple classifier – not as simple as with OneR – but it could be the take home message from running this algorithm on this data – if you are satisfied with the results! outlook = sunny | temperature = hot | | windy = TRUE: yes | | windy = FALSE: no | temperature = mild: yes | temperature = cool: yes outlook = overcast | temperature = hot: no | temperature = mild: yes | temperature = cool: yes outlook = rainy: no**Let’s Try WEKA ID3 on njcrimenominal**Seems to noticeably improve on our very simple methods on this slightly more interesting dataset • Try 10-fold unemploy = hi: bad unemploy = med | education = hi: ok | education = med | | twoparent = hi: null | | twoparent = med: bad | | twoparent = low: ok | education = low | | pop = hi: null | | pop = med: ok | | pop = low: bad unemploy = low: ok === Confusion Matrix === a b <-- classified as 5 2 | a = bad 3 22 | b = ok**Another Thing or Two**• Using this method, if an attribute is essentially a primary key – identifying the instances, dividing based on it will give the maximum information gain because no further information is needed to determine the class (the entropy will be 0) • However, branching based on a key is not interesting, nor is useful for predicting future un-trained on instances • The more general idea is that the entropy measure favors splitting using attributes with more possible values • A common adjustment is to use a “gain ratio” …**Gain Ratio**• Take info gain and divide by “intrinsic info” from the split • E.g. top split above**Still not a slam dunk**• Even this gets adjusted in some approaches to avoid yet other problems (see p 97)**ID3 in context**• Quinlan (1986) published about ID3 as first major successful decision tree learner in Machine Learning • He continued to improve the algorithm • His C4.5 was published in a book, and is available as J48 in WEKA • Improvements included dealing with numeric attributes, missing values, noisy data, and generating rules from trees (see Section 6.1) • His further efforts were commercial and proprietary instead of published in research literature • Probably almost every graduate student in Machine Learning starts out by writing a version of ID3 so that they FULLY understand it**Class Exercise**• ID3 cannot run on jappanbank data since it includes some numeric attributes • Let’s run WEKA J48 on japanbank

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