Handling Numeric Attributes in Hoeffding Trees Bernhard Pfahringer, Geoff Holmes and Richard Kirkby

Handling Numeric Attributes in Hoeffding treesPfahringer, Holmes and Kirkby - Machine Learning Group Overview Hoeffding trees are excellent for classification tasks on data streams. Handling numeric attributes well is crucial to performance in conventional decision trees (for example, C4.5 -> C4.8) Does handling numeric attributes matter for streamed data? We implement a range of methods and empirically evaluate their accuracy and costs.

Handling Numeric Attributes in Hoeffding treesPfahringer, Holmes and Kirkby - Machine Learning Group Data Streams - reminder Idea is that data is being provided from a continuous source: –Examples processed one at a time (inspected once) –Memory is limited (!) –Model construction must scale (NlogN in num examples) –Be ready to predict at any time As memory is limited this will have implications for any numeric handling method you might construct Only consider methods that work as the tree is built

Handling Numeric Attributes in Hoeffding treesPfahringer, Holmes and Kirkby - Machine Learning Group Main assumptions/limitations Assume a stationary concept, i.e. no concept drift or change –may seem very limiting, but … Three-way trade-off: –memory –speed –accuracy Used only artificial data sources

Handling Numeric Attributes in Hoeffding treesPfahringer, Holmes and Kirkby - Machine Learning Group Hoeffding Trees Introduced by Domingos and Hulten (VFDT) “Extension” of decision trees to streams HT Algorithm: –Init tree T to root node –For each example from stream Find leaf L for this example Update counts in L with attr values of example and compute split function (eg Info Gain, IG) for each attribute If IG(best attr) – IG(next best attr) > ε then split L on best attr

Handling Numeric Attributes in Hoeffding treesPfahringer, Holmes and Kirkby - Machine Learning Group Active leaf data structure For each class value: –for each nominal attribute: for each possible value: –keep sum of counts/weights –for each numeric attribute: keep sufficient stats to approximate the distribution various possibilities: here assume normal distribution so estimate/record: n,mean,variance, + min/max

Handling Numeric Attributes in Hoeffding treesPfahringer, Holmes and Kirkby - Machine Learning Group Numeric Handling Methods VFDT (VFML – Hulten & Domingos, 2003) –Summarize the numeric distribution with a histogram made up of a maximum number of bins N (default 1000) –Bin boundaries determined by first N unique values seen in the stream. –Issues: method sensitive to data order and choosing a good N for a particular problem Exhaustive Binary Tree (BINTREE – Gama et al, 2003) –Closest implementation of a batch method –Incrementally update a binary tree as data is observed –Issues: high memory cost, high cost of split search, data order

Handling Numeric Attributes in Hoeffding treesPfahringer, Holmes and Kirkby - Machine Learning Group Numeric Handling Methods Quantile Summaries (GK – Greenwald and Khanna, 2001) –Motivation comes from VLDB –Maintain sample of values (quantiles) plus range of possible ranks that the samples can take (tuples) –Extremely space efficient –Issues: use max number of tuples per summary

Handling Numeric Attributes in Hoeffding treesPfahringer, Holmes and Kirkby - Machine Learning Group Handling Numeric Methods Gaussian Approximation (GAUSS) –Assume values conform to Normal Distribution –Maintain five numbers (eg mean, variance, weight, max, min) –Note: not sensitive to data order –Incrementally updateable –Using the max, min information per class – split the range into N equal parts –For each part use the 5 numbers per class to compute the approx class distribution Use the above to compute the IG of that split

Handling Numeric Attributes in Hoeffding treesPfahringer, Holmes and Kirkby - Machine Learning Group Gaussian approximation – 2 class problem

Handling Numeric Attributes in Hoeffding treesPfahringer, Holmes and Kirkby - Machine Learning Group Gaussian approximation – 3 class problem

Handling Numeric Attributes in Hoeffding treesPfahringer, Holmes and Kirkby - Machine Learning Group Gaussian approximation – 4 class problem

Handling Numeric Attributes in Hoeffding treesPfahringer, Holmes and Kirkby - Machine Learning Group Empirical Evaluation Use each numeric handling method (8 in total) to build a Hoeffding Tree (HTMC) Vary parameters of some methods (VFML10,100,1000; BT; GK100,1000; GAUSS10,100) Train models for 10 hours – then test on one million (holdout) examples Define three application scenarios –Sensor network (100K memory limit) –Handheld (32MB) –Server (400MB)

Handling Numeric Attributes in Hoeffding treesPfahringer, Holmes and Kirkby - Machine Learning Group Data generators Random tree (Domingos&Hulten): –(RTS) 10 num, 10 nom 5 values, 2 classes, leaves start at level 3, max level 5, plus version with 10% noise added (RTSN) –(RTC) 50 num, 50 nom 5 values, 2 classes, leaves start at level 5, max level 10, plus version with 10% noise added (RTCN) Random RBF (Kirkby): –(RRBFS) 10 num, 100 centers, 2 classes –(RRBFC) 50 num, 1000 centers, 2 classes Waveform (Aha): –(Wave21): 21 noisy num, (Wave40): +19 irrelevant num; 3 classes (GenF1-GenF10) (Agrawal etal): –hypothetical loan applications, 10 different rule(s) over 6 num + 3 nom attrs, 5% noise, 2 classes

Handling Numeric Attributes in Hoeffding treesPfahringer, Holmes and Kirkby - Machine Learning Group Tree Measurements Accuracy (% correct) Number of training examples processed in 10 hours (in millions) Number of active leaves (in hundreds) Number of inactive leaves (in hundreds) Total nodes (in hundreds) Tree depth Training speed (% of generation speed) Prediction speed (% of generation speed)

Handling Numeric Attributes in Hoeffding treesPfahringer, Holmes and Kirkby - Machine Learning Group Sensor Network (100K memory limit) Method% correct Train (million) Active Leaves Inactive (hdrds) Total Nodes AvgTree Depth Train Spd % Pred Spd % VF VF VF BT GK GK GAUSS GAUSS

Handling Numeric Attributes in Hoeffding treesPfahringer, Holmes and Kirkby - Machine Learning Group Handheld Environment (32MB memory limit) Method% correct Train (million) Active Leaves Inactive (hdrds) Total Nodes AvgTree Depth Train Spd % Pred Spd % VF VF VF BT GK GK GAUSS GAUSS

Handling Numeric Attributes in Hoeffding treesPfahringer, Holmes and Kirkby - Machine Learning Group Server Environment (400MB memory limit) Method% correct Train (million) Active Leaves Inactive (hdrds) Total Nodes AvgTree Depth Train Spd % Pred Spd % VF VF VF BT GK GK GAUSS GAUSS

Handling Numeric Attributes in Hoeffding treesPfahringer, Holmes and Kirkby - Machine Learning Group Overall results - comments VFML10 is superior on average in all environments, followed closely by GAUSS10 GK methods are generally competitive BINTREE is only competitive in a server setting Default setting of 1000 for VFML is a poor choice Crude binning provides more space which leads to faster growth and better trees (more room to grow) Higher values for GAUSS leads to very deep trees (in excess of the # of attributes) suggesting repeated splitting (too fine grained)

Handling Numeric Attributes in Hoeffding treesPfahringer, Holmes and Kirkby - Machine Learning Group Remarks – sensor network environment Number of training examples low because learning stops when last active leaf is deactivated (mem mgmt freezes nodes – low # examples, low probability of splitting) Most accurate methods VFML10, GAUSS10

Handling Numeric Attributes in Hoeffding treesPfahringer, Holmes and Kirkby - Machine Learning Group Remarks – Handheld Environment Generates smaller trees (than server) and can therefore process more examples

Handling Numeric Attributes in Hoeffding treesPfahringer, Holmes and Kirkby - Machine Learning Group Remarks – Server Environment

Handling Numeric Attributes in Hoeffding treesPfahringer, Holmes and Kirkby - Machine Learning Group VFML10 vs GAUSS10 – Closer Analysis Recall VFML10 is superior on average Sensor (avg 87.7 vs 86.2) –GAUSS10 superior on 10 –VFML10 superior on 6 (2 no difference) Handheld (avg 91.5 vs 91.4) –GAUSS10 superior on 4 –VFML10 superior on 8 (6 no difference) Server (avg 91.4 vs 91.2) –GAUSS10 superior on 6 –VFML10 superior on 6 (6 no difference)

Handling Numeric Attributes in Hoeffding treesPfahringer, Holmes and Kirkby - Machine Learning Group Data order

Handling Numeric Attributes in Hoeffding treesPfahringer, Holmes and Kirkby - Machine Learning Group Conclusion We have presented a method for handling numeric attributes in data streams that performs well in empirical studies The methods employing the most approximation were superior – they allow greater growth when memory is limited. On a dataset by dataset analysis there is not much to choose between VFML10 and GAUSS10 Gains made in handling numeric variables come at a cost in terms of training and prediction speed – the cost is high in some environments

Handling Numeric Attributes in Hoeffding treesPfahringer, Holmes and Kirkby - Machine Learning Group All algorithms available All methods and an environment for experimental evaluation of data streams is available from the above URL – system is called Massive Online Analysis (MOA)