Data Mining: Classification
Classification and Prediction What is classification? What is prediction? Issues regarding classification and prediction Classification by decision tree induction Bayesian Classification Classification by backpropagation Classification based on concepts from association rule mining Other Classification Methods Prediction Classification accuracy Summary
Classification vs. Prediction Classification: - predicts categorical class labels
- classifies data (constructs a model) based on the training set and the values (class labels) in a classifying attribute and uses it in classifying new data
Prediction: - models continuous-valued functions, i.e., predicts unknown or missing values
Typical Applications - credit approval
- target marketing
- medical diagnosis
- treatment effectiveness analysis
Classification—A Two-Step Process Model construction: describing a set of predetermined classes - Each tuple/sample is assumed to belong to a predefined class, as determined by the class label attribute
- The set of tuples used for model construction: training set
- The model is represented as classification rules, decision trees, or mathematical formulae
Model usage: for classifying future or unknown objects - Estimate accuracy of the model
- The known label of test sample is compared with the classified result from the model
- Accuracy rate is the percentage of test set samples that are correctly classified by the model
- Test set is independent of training set, otherwise over-fitting will occur
Classification Process (1): Model Construction
Classification Process (2): Use the Model in Prediction
Supervised vs. Unsupervised Learning Supervised learning (classification) - Supervision: The training data (observations, measurements, etc.) are accompanied by labels indicating the class of the observations
- New data is classified based on the training set
Unsupervised learning (clustering) - The class labels of training data is unknown
- Given a set of measurements, observations, etc. with the aim of establishing the existence of classes or clusters in the data
Classification and Prediction What is classification? What is prediction? Issues regarding classification and prediction Classification by decision tree induction Bayesian Classification Classification by backpropagation Classification based on concepts from association rule mining Other Classification Methods Prediction Classification accuracy Summary
Issues (1): Data Preparation Data cleaning - Preprocess data in order to reduce noise and handle missing values
Relevance analysis (feature selection) - Remove the irrelevant or redundant attributes
Data transformation - Generalize and/or normalize data
Issues (2): Evaluating Classification Methods Predictive accuracy Speed and scalability - time to construct the model
- time to use the model
Robustness - handling noise and missing values
Scalability - efficiency in disk-resident databases
Interpretability: - understanding and insight provded by the model
Goodness of rules - decision tree size
- compactness of classification rules
Classification and Prediction What is classification? What is prediction? Issues regarding classification and prediction Classification by decision tree induction Bayesian Classification Classification by backpropagation Classification based on concepts from association rule mining Other Classification Methods Prediction Classification accuracy Summary
Classification by Decision Tree Induction Decision tree - A flow-chart-like tree structure
- Internal node denotes a test on an attribute
- Branch represents an outcome of the test
- Leaf nodes represent class labels or class distribution
Decision tree generation consists of two phases - Tree construction
- At start, all the training examples are at the root
- Partition examples recursively based on selected attributes
- Tree pruning
- Identify and remove branches that reflect noise or outliers
Use of decision tree: Classifying an unknown sample - Test the attribute values of the sample against the decision tree
Training Dataset
Output: A Decision Tree for “buys_computer”
Algorithm for Decision Tree Induction Basic algorithm (a greedy algorithm) - Tree is constructed in a top-down recursive divide-and-conquer manner
- At start, all the training examples are at the root
- Attributes are categorical (if continuous-valued, they are discretized in advance)
- Examples are partitioned recursively based on selected attributes
- Test attributes are selected on the basis of a heuristic or statistical measure (e.g., information gain)
Conditions for stopping partitioning - All samples for a given node belong to the same class
- There are no remaining attributes for further partitioning – majority voting is employed for classifying the leaf
- There are no samples left
Attribute Selection Measure Information gain (ID3/C4.5) - All attributes are assumed to be categorical
- Can be modified for continuous-valued attributes
Gini index (IBM IntelligentMiner) - All attributes are assumed continuous-valued
- Assume there exist several possible split values for each attribute
- May need other tools, such as clustering, to get the possible split values
- Can be modified for categorical attributes
Information Gain (ID3/C4.5) Select the attribute with the highest information gain Assume there are two classes, P and N - Let the set of examples S contain p elements of class P and n elements of class N
- The amount of information, needed to decide if an arbitrary example in S belongs to P or N is defined as
Information Gain in Decision Tree Induction Assume that using attribute A a set S will be partitioned into sets {S1, S2 , …, Sv} - If Si contains pi examples of P and ni examples of N, the entropy, or the expected information needed to classify objects in all subtrees Si is
The encoding information that would be gained by branching on A
Attribute Selection by Information Gain Computation Class P: buys_computer = “yes” Class N: buys_computer = “no” I(p, n) = I(9, 5) =0.940 Compute the entropy for age:
Gini Index (IBM IntelligentMiner) If a data set T contains examples from n classes, gini index, gini(T) is defined as where pj is the relative frequency of class j in T. If a data set T is split into two subsets T1 and T2 with sizes N1 and N2 respectively, the gini index of the split data contains examples from n classes, the gini index gini(T) is defined as The attribute provides the smallest ginisplit(T) is chosen to split the node (need to enumerate all possible splitting points for each attribute).
Extracting Classification Rules from Trees Represent the knowledge in the form of IF-THEN rules One rule is created for each path from the root to a leaf Each attribute-value pair along a path forms a conjunction The leaf node holds the class prediction Rules are easier for humans to understand Example - IF age = “<=30” AND student = “no” THEN buys_computer = “no”
- IF age = “<=30” AND student = “yes” THEN buys_computer = “yes”
- IF age = “31…40” THEN buys_computer = “yes”
- IF age = “>40” AND credit_rating = “excellent” THEN buys_computer = “yes”
- IF age = “>40” AND credit_rating = “fair” THEN buys_computer = “no”
Avoid Overfitting in Classification The generated tree may overfit the training data - Too many branches, some may reflect anomalies due to noise or outliers
- Result is in poor accuracy for unseen samples
Two approaches to avoid overfitting - Prepruning: Halt tree construction early—do not split a node if this would result in the goodness measure falling below a threshold
- Difficult to choose an appropriate threshold
- Postpruning: Remove branches from a “fully grown” tree—get a sequence of progressively pruned trees
- Use a set of data different from the training data to decide which is the “best pruned tree”
Approaches to Determine the Final Tree Size Separate training (2/3) and testing (1/3) sets Use cross validation, e.g., 10-fold cross validation Use all the data for training - but apply a statistical test (e.g., chi-square) to estimate whether expanding or pruning a node may improve the entire distribution
Use minimum description length (MDL) principle: - halting growth of the tree when the encoding is minimized
Enhancements to basic decision tree induction Allow for continuous-valued attributes - Dynamically define new discrete-valued attributes that partition the continuous attribute value into a discrete set of intervals
Handle missing attribute values - Assign the most common value of the attribute
- Assign probability to each of the possible values
Attribute construction - Create new attributes based on existing ones that are sparsely represented
- This reduces fragmentation, repetition, and replication
Classification in Large Databases Classification—a classical problem extensively studied by statisticians and machine learning researchers Scalability: Classifying data sets with millions of examples and hundreds of attributes with reasonable speed Why decision tree induction in data mining? - relatively faster learning speed (than other classification methods)
- convertible to simple and easy to understand classification rules
- can use SQL queries for accessing databases
- comparable classification accuracy with other methods
Scalable Decision Tree Induction Methods in Data Mining Studies SLIQ (EDBT’96 — Mehta et al.) - builds an index for each attribute and only class list and the current attribute list reside in memory
SPRINT (VLDB’96 — J. Shafer et al.) - constructs an attribute list data structure
PUBLIC (VLDB’98 — Rastogi & Shim) - integrates tree splitting and tree pruning: stop growing the tree earlier
RainForest (VLDB’98 — Gehrke, Ramakrishnan & Ganti) - separates the scalability aspects from the criteria that determine the quality of the tree
- builds an AVC-list (attribute, value, class label)
Data Cube-Based Decision-Tree Induction Integration of generalization with decision-tree induction (Kamber et al’97). Classification at primitive concept levels - E.g., precise temperature, humidity, outlook, etc.
- Low-level concepts, scattered classes, bushy classification-trees
- Semantic interpretation problems.
Cube-based multi-level classification - Relevance analysis at multi-levels.
- Information-gain analysis with dimension + level.
Presentation of Classification Results
Classification and Prediction What is classification? What is prediction? Issues regarding classification and prediction Classification by decision tree induction Bayesian Classification Classification by backpropagation Classification based on concepts from association rule mining Other Classification Methods Prediction Classification accuracy Summary
Bayesian Classification: Why? Probabilistic learning: Calculate explicit probabilities for hypothesis, among the most practical approaches to certain types of learning problems Incremental: Each training example can incrementally increase/decrease the probability that a hypothesis is correct. Prior knowledge can be combined with observed data. Probabilistic prediction: Predict multiple hypotheses, weighted by their probabilities Standard: Even when Bayesian methods are computationally intractable, they can provide a standard of optimal decision making against which other methods can be measured
Bayesian Theorem Given training data D, posteriori probability of a hypothesis h, P(h|D) follows the Bayes theorem MAP (maximum posteriori) hypothesis Practical difficulty: require initial knowledge of many probabilities, significant computational cost
Naïve Bayes Classifier (I) A simplified assumption: attributes are conditionally independent: Greatly reduces the computation cost, only count the class distribution.
Naive Bayesian Classifier (II) Given a training set, we can compute the probabilities
Bayesian classification The classification problem may be formalized using a-posteriori probabilities: P(C|X) = prob. that the sample tuple X= is of class C. E.g. P(class=N | outlook=sunny,windy=true,…) Idea: assign to sample X the class label C such that P(C|X) is maximal
Estimating a-posteriori probabilities Bayes theorem: P(C|X) = P(X|C)·P(C) / P(X) P(X) is constant for all classes P(C) = relative freq of class C samples C such that P(C|X) is maximum = C such that P(X|C)·P(C) is maximum Problem: computing P(X|C) is unfeasible!
Naïve Bayesian Classification Naïve assumption: attribute independence P(x1,…,xk|C) = P(x1|C)·…·P(xk|C) If i-th attribute is categorical: P(xi|C) is estimated as the relative freq of samples having value xi as i-th attribute in class C If i-th attribute is continuous: P(xi|C) is estimated thru a Gaussian density function Computationally easy in both cases
Play-tennis example: estimating P(xi|C)
Play-tennis example: classifying X An unseen sample X = P(X|p)·P(p) = P(rain|p)·P(hot|p)·P(high|p)·P(false|p)·P(p) = 3/9·2/9·3/9·6/9·9/14 = 0.010582 P(X|n)·P(n) = P(rain|n)·P(hot|n)·P(high|n)·P(false|n)·P(n) = 2/5·2/5·4/5·2/5·5/14 = 0.018286 Sample X is classified in class n (don’t play)
The independence hypothesis… … makes computation possible … yields optimal classifiers when satisfied … but is seldom satisfied in practice, as attributes (variables) are often correlated. Attempts to overcome this limitation: - Bayesian networks, that combine Bayesian reasoning with causal relationships between attributes
- Decision trees, that reason on one attribute at the time, considering most important attributes first
Bayesian Belief Networks (I)
Bayesian Belief Networks (II) Bayesian belief network allows a subset of the variables conditionally independent A graphical model of causal relationships Several cases of learning Bayesian belief networks - Given both network structure and all the variables: easy
- Given network structure but only some variables
- When the network structure is not known in advance
Classification and Prediction What is classification? What is prediction? Issues regarding classification and prediction Classification by decision tree induction Bayesian Classification Classification by backpropagation Classification based on concepts from association rule mining Other Classification Methods Prediction Classification accuracy Summary
Neural Networks Advantages - prediction accuracy is generally high
- robust, works when training examples contain errors
- output may be discrete, real-valued, or a vector of several discrete or real-valued attributes
- fast evaluation of the learned target function
Criticism - long training time
- difficult to understand the learned function (weights)
- not easy to incorporate domain knowledge
A Neuron The n-dimensional input vector x is mapped into variable y by means of the scalar product and a nonlinear function mapping
Network Training The ultimate objective of training - obtain a set of weights that makes almost all the tuples in the training data classified correctly
Steps - Initialize weights with random values
- Feed the input tuples into the network one by one
- For each unit
- Compute the net input to the unit as a linear combination of all the inputs to the unit
- Compute the output value using the activation function
- Compute the error
- Update the weights and the bias
Multi-Layer Perceptron
Network Pruning and Rule Extraction Network pruning - Fully connected network will be hard to articulate
- N input nodes, h hidden nodes and m output nodes lead to h(m+N) weights
- Pruning: Remove some of the links without affecting classification accuracy of the network
Extracting rules from a trained network - Discretize activation values; replace individual activation value by the cluster average maintaining the network accuracy
- Enumerate the output from the discretized activation values to find rules between activation value and output
- Find the relationship between the input and activation value
- Combine the above two to have rules relating the output to input
Classification and Prediction What is classification? What is prediction? Issues regarding classification and prediction Classification by decision tree induction Bayesian Classification Classification by backpropagation Classification based on concepts from association rule mining Other Classification Methods Prediction Classification accuracy Summary
Association-Based Classification Several methods for association-based classification - ARCS: Quantitative association mining and clustering of association rules (Lent et al’97)
- It beats C4.5 in (mainly) scalability and also accuracy
- Associative classification: (Liu et al’98)
- It mines high support and high confidence rules in the form of “cond_set => y”, where y is a class label
- CAEP (Classification by aggregating emerging patterns) (Dong et al’99)
- Emerging patterns (EPs): the itemsets whose support increases significantly from one class to another
- Mine Eps based on minimum support and growth rate
Classification and Prediction What is classification? What is prediction? Issues regarding classification and prediction Classification by decision tree induction Bayesian Classification Classification by backpropagation Classification based on concepts from association rule mining Other Classification Methods Prediction Classification accuracy Summary
Other Classification Methods k-nearest neighbor classifier case-based reasoning Genetic algorithm Rough set approach Fuzzy set approaches
Instance-Based Methods Instance-based learning: - Store training examples and delay the processing (“lazy evaluation”) until a new instance must be classified
Typical approaches - k-nearest neighbor approach
- Instances represented as points in a Euclidean space.
- Locally weighted regression
- Constructs local approximation
- Case-based reasoning
- Uses symbolic representations and knowledge-based inference
The k-Nearest Neighbor Algorithm All instances correspond to points in the n-D space. The nearest neighbor are defined in terms of Euclidean distance. The target function could be discrete- or real- valued. For discrete-valued, the k-NN returns the most common value among the k training examples nearest to xq. Vonoroi diagram: the decision surface induced by 1-NN for a typical set of training examples.
Discussion on the k-NN Algorithm The k-NN algorithm for continuous-valued target functions - Calculate the mean values of the k nearest neighbors
Distance-weighted nearest neighbor algorithm - Weight the contribution of each of the k neighbors according to their distance to the query point xq
- giving greater weight to closer neighbors
- Similarly, for real-valued target functions
Robust to noisy data by averaging k-nearest neighbors Curse of dimensionality: distance between neighbors could be dominated by irrelevant attributes. - To overcome it, axes stretch or elimination of the least relevant attributes.
Case-Based Reasoning Also uses: lazy evaluation + analyze similar instances Difference: Instances are not “points in a Euclidean space” Example: Water faucet problem in CADET (Sycara et al’92) Methodology - Instances represented by rich symbolic descriptions (e.g., function graphs)
- Multiple retrieved cases may be combined
- Tight coupling between case retrieval, knowledge-based reasoning, and problem solving
Research issues - Indexing based on syntactic similarity measure, and when failure, backtracking, and adapting to additional cases
Remarks on Lazy vs. Eager Learning Instance-based learning: lazy evaluation Decision-tree and Bayesian classification: eager evaluation Key differences - Lazy method may consider query instance xq when deciding how to generalize beyond the training data D
- Eager method cannot since they have already chosen global approximation when seeing the query
Efficiency: Lazy - less time training but more time predicting Accuracy - Lazy method effectively uses a richer hypothesis space since it uses many local linear functions to form its implicit global approximation to the target function
- Eager: must commit to a single hypothesis that covers the entire instance space
Genetic Algorithms GA: based on an analogy to biological evolution Each rule is represented by a string of bits An initial population is created consisting of randomly generated rules - e.g., IF A1 and Not A2 then C2 can be encoded as 100
Based on the notion of survival of the fittest, a new population is formed to consists of the fittest rules and their offsprings The fitness of a rule is represented by its classification accuracy on a set of training examples Offsprings are generated by crossover and mutation
Rough Set Approach Rough sets are used to approximately or “roughly” define equivalent classes A rough set for a given class C is approximated by two sets: a lower approximation (certain to be in C) and an upper approximation (cannot be described as not belonging to C) Finding the minimal subsets (reducts) of attributes (for feature reduction) is NP-hard but a discernibility matrix is used to reduce the computation intensity
Fuzzy Sets Fuzzy logic uses truth values between 0.0 and 1.0 to represent the degree of membership (such as using fuzzy membership graph) Attribute values are converted to fuzzy values - e.g., income is mapped into the discrete categories {low, medium, high} with fuzzy values calculated
For a given new sample, more than one fuzzy value may apply Each applicable rule contributes a vote for membership in the categories Typically, the truth values for each predicted category are summed
Classification and Prediction What is classification? What is prediction? Issues regarding classification and prediction Classification by decision tree induction Bayesian Classification Classification by backpropagation Classification based on concepts from association rule mining Other Classification Methods Prediction Classification accuracy Summary
What Is Prediction? Prediction is similar to classification - First, construct a model
- Second, use model to predict unknown value
- Major method for prediction is regression
- Linear and multiple regression
- Non-linear regression
Prediction is different from classification - Classification refers to predict categorical class label
- Prediction models continuous-valued functions
Predictive Modeling in Databases Predictive modeling: Predict data values or construct generalized linear models based on the database data. One can only predict value ranges or category distributions Method outline: - Minimal generalization
- Attribute relevance analysis
- Generalized linear model construction
- Prediction
Determine the major factors which influence the prediction - Data relevance analysis: uncertainty measurement, entropy analysis, expert judgement, etc.
Multi-level prediction: drill-down and roll-up analysis
Regress Analysis and Log-Linear Models in Prediction Linear regression: Y = + X - Two parameters , and specify the line and are to be estimated by using the data at hand.
- using the least squares criterion to the known values of Y1, Y2, …, X1, X2, ….
Multiple regression: Y = b0 + b1 X1 + b2 X2. - Many nonlinear functions can be transformed into the above.
Log-linear models: - The multi-way table of joint probabilities is approximated by a product of lower-order tables.
- Probability: p(a, b, c, d) = ab acad bcd
Prediction: Numerical Data
Prediction: Categorical Data
Classification and Prediction What is classification? What is prediction? Issues regarding classification and prediction Classification by decision tree induction Bayesian Classification Classification by backpropagation Classification based on concepts from association rule mining Other Classification Methods Prediction Classification accuracy Summary
Classification Accuracy: Estimating Error Rates |