Selekcja informacji dla analizy danych z mikromacierzy Włodzisław Duch, Jacek Biesiada Dept. of Informatics, Nicolaus Copernicus University, Google: Duch BIT 2007

MikromacierzeMikromacierze Sondy wykrywające przez hybrydyzację komplementarne DNA/RNA

Selection of information Attention: basic cognitive skill, focus, constrained activationsAttention: basic cognitive skill, focus, constrained activations Find relevant information:Find relevant information: –discard attributes that do not contain information, –use weights to express the relative importance, –create new, more informative attributes –reduce dimensionality aggregating information Ranking: treat each feature as independent.Ranking: treat each feature as independent. Selection: search for subsets, remove redundant.Selection: search for subsets, remove redundant. Filters: universal criteria, model-independent.Filters: universal criteria, model-independent. Wrappers: criteria specific for data models are used.Wrappers: criteria specific for data models are used. Frapper: filter + wrapper in the final stage.Frapper: filter + wrapper in the final stage.

Filters & Wrappers Filter approach: define your problem, for example assignment of class labels; define an index of relevance for each feature R(X i ) rank features according to their relevance R(X i1 )  R(X i2 ) .. R(X id ) rank all features with relevance above threshold R(X i1 )  t R Wrapper approach: select predictor P and performance measure P(Data) define search scheme: forward, backward or mixed selection evaluate starting subset of features X s, ex. single best or all features add/remove feature X i, accept new set X s  {X s +X i } if P(Data| X s +X i )>P(Data| X s )

Information theory - filters X – vectors, X j – attributes, X j =f attribute values, C i - class i =1.. K, joint probability distribution p(C, X j ). The amount of information contained in this joint distribution, summed over all classes, gives an estimation of feature importance: For continuous attribute values integrals are approximated by sums. This implies discretization into r k (f) regions, an issue in itself. Alternative: fitting p(C i,f) density using Gaussian or other kernels. Which method is more accurate and what are expected errors?

Information gain Information gained by considering the joint probability distribution p(C, f) is a difference between: A feature is more important if its information gain is larger. Modifications of the information gain, frequently used as criteria in decision trees, include: IGR(C,X j ) = IG(C,X j )/I(X j ) the gain ratio IGn(C,X j ) = IG(C,X j )/I(C) an asymmetric dependency coefficient D M (C,X j ) =  IG(C,X j )/I(C,X j ) normalized Mantaras distance

Information indices Information gained considering attribute X j and classes C together is also known as ‘mutual information’, equal to the Kullback-Leibler divergence between joint and product probability distributions: Entropy distance measure is a sum of conditional information: Symmetrical uncertainty coefficient is obtained from entropy distance:

Purity indices Many information-based quantities may be used to evaluate attributes. Consistency or purity-based indices are one alternative. For selection of subset of attributes F={X i } the sum runs over all Cartesian products, or multidimensional partitions r k (F). Advantages: simplest approach both ranking and selection Hashing techniques are used to calculate p(r k (F)) probabilities.

Correlation coefficient Perhaps the simplest index is based on the Pearson’s correlation coefficient (CC) that calculates expectation values for product of feature values and class values: For feature values that are linearly dependent correlation coefficient is  or  ; for complete independence of class and X j distribution CC= 0. How significant are small correlations? It depends on the number of samples n. The answer (see Numerical Recipes is given by: For n=1000 even small CC=0.02 gives P ~ 0.5, but for n=10 such CC gives only P ~ 0.05.

Other relevance indices Mutual information is based on Kullback-Leibler distance, any distance measure between distributions may also be used, ex. Jeffreys-Matusita Bayesian concentration measure is simply: Many other such dissimilarity measures exist. Which is the best? In practice they all are similar, although accuracy of calculation of indices is important; relevance indices should be insensitive to noise and unbiased in their treatment of features with many values; IT is fine.

Discretization All indices of feature relevance require summation over probability distributions. What to do if the feature is continuous? There are two solutions: 2.Discretize, put feature values in some bins, and calculate sums. Histograms with equal width of bins (intervals); Histograms with equal number of samples per bin; Maxdiff histograms: bins starting in the middle (x i+1  x i )/2 of largest gaps V-optimal: sum of variances in bins should be minimal (good but costly). 1. Fit some functions to histogram distributions using Parzen windows, ex. fit a sum of several Gaussians, and integrate to get indices, ex:

Discretized information With partition of X j feature values x into r k bins, joint information is calculated as: and mutual information as:

Tree (entropy-based) discretization V-opt histograms are good, but difficult to create (dynamic programming techniques should be used). Simple approach: use decision trees with a single feature, or a small subset of features, to find good splits – this avoids local discretization. Example: C4.5 decision tree discretization maximizing information gain, or SSV tree based separability criterion, vs. constant width bins. Hypothyroid screening data, 5 continuous features, MI shown. Compare the amount of mutual information or the correlation between class labels and discretized values of features using equal width discretization and decision tree discretization into 4, bins.

Ranking algorithms Based on: MI(C,f) : mutual information (information gain) ICR(C|f) : information gain ratio IC(C|f) : information from max C posterior distribution GD(C,f) : transinformation matrix with Mahalanobis distance JBC(C,f) : Bayesian purity index + 7 other methods based on IC and correlation-based distances Relieff selection methods. Several new simple indices; estimation errors and convergence. Markov blankets Boosting Margins

Selection algorithms Maximize evaluation criterion for single & remove redundant features. 1. MI(C;f)  MI(f;g) algorithm 2. IC(C,f)  IC(f,g), same algorithm but with IC criterion 3. Max IC(C;F) adding single attribute that maximizes IC 4. Max MI(C;F) adding single attribute that maximizes IC 5. SSV decision tree based on separability criterion.

4 Gaussians in 8D Artificial data: set of 4 Gaussians in 8D, 1000 points per Gaussian, each as a separate class. Dimension 1-4, independent, Gaussians centered at: (0,0,0,0), (2,1,0.5,0.25), (4,2,1,0.5), (6,3,1.5,0.75). Ranking and overlapping strength are inversely related: Ranking: X 1  X 2  X 3  X 4. Attributes X i+4 = 2X i + uniform noise ±1.5. Best ranking: X 1  X 5  X 2  X 6  X 3  X 7  X 4  X 8 Best selection: X 1  X 2  X 3  X 4  X 5  X 6  X 7  X 8

Dim X 1 vs. X 2

Dim X 1 vs. X 5

Ranking for 8D Gaussians Partitions of each attribute into 4, 8, 16, 24, 32 parts, with equal width. Methods that found perfect ranking: MI(C;f), IGR(C;f), WI(C,f), GD transinformation distance IC(f) : correct, except for P8, feature 2-6 reversed (6 is the noisy version of 2). Other, more sophisticated algorithms, made more errors. Selection for Gaussian distributions is rather easy using any evaluation measure. Simpler algorithms work better.

Selection for 8D Gaussians Partitions of each attribute into 4, 8, 16, 24, 32 parts, with equal width. Ideal selection: subsets with {1}, {1+2}, {1+2+3}, or { } attributes. 1. MI(C;f)  MI(f;g) algorithm: P24 no errors, for P8, 16, 32 small error (4  8) 2.Max MI(C;F): P8-24 no errors, P32 (3,4  7,8) 3.Max IC(C;F) : P24 no errors, P8 (2  6), P16 (3  7), P32 (3,4  7,8) 4.SSV decision tree based on separability criterion: creates its own discretization. Selects 1, 2, 6, 3, 7, others are not important. Univariate trees have bias for slanted distributions. Selection should take into account the type of classification system that will be used.

Discretization example MI index for 5 continuous features of the hypothyroid screening data. EP = equal width partition into 4, bins SSV = decision tree partition (discretization) into 4, bins

Hypothyroid: equal bins Mutual information for different number of equal width partitions, ordered from largest to smallest, for the hypothyroid data: 6 continuous and 15 binary attributes.

Hypothyroid: SSV bins Mutual information for different number of equal SSV decision tree partitions, ordered from largest to smallest, for the hypothyroid data. Values are twice as large since bins are more pure.

Hypothyroid: ranking Best ranking: largest area under curve: accuracy(best n features). SBL: evaluating and adding one attribute at a time (costly). Best 2: SBL, best 3: SSV BFS, best 4: SSV beam; BA - failure

Hypothyroid: ranking Results from FSM neurofuzzy system. Best 2: SBL, best 3: SSV BFS, best 4: SSV beam; BA – failure Global correlation misses local usefulness...

Hypothyroid: SSV ranking More results using FSM and selection based on SSV. SSV with beam search P24 finds the best small subsets, depending on the search depth; here best results for 5 attributes are achieved.

Leukemia: Bayes rules Top: test, bottom: train; green = p(C|X) for Gaussian-smoothed density with  =0.01, 0.02, 0.05, 0.20 (Zyxin).

Leukemia boosting 3 best genes, evaluation using bootstrap.

Leukemia boosting 3 best genes, evaluation using bootstrap.

Leukemia SVM LVO Problems with stability

GM example, Leukemia data Two types of leukemia, ALL and AML, 7129 genes, 38 train/34 test. Try SSV – decision trees are usually fast, no preprocessing. 1. Run SSV selecting 3 CVs; train accuracy is 100%, test 91% (3 err) and only 1 feature F4847 is used. 2. Removing it manually and running SSV creates a tree with F2020, 1 err train, 9 err test. This is not very useful SSV Stump Field shows ranking of 1-level trees, SSV may also be used for selection, but trees separate training data with F Standardize all the data, run SVM, 100% with all 38 as support vectors but test is 59% (14 errors). 5. Use SSV for selection of 10 features, run SVM with these features, train 100%, test 97% (1 error only). Small samples... but a profile of genes should be sufficient.

ConclusionsConclusions About 20 ranking and selection methods have been checked. The actual feature evaluation index (information, consistency, correlation etc) is not so important. Discretization is very important; naive equi-width or equidistance discretization may give unpredictable results; entropy-based discretization is fine, but separability-based is less expensive. Continuous kernel-based approximations to calculation of feature evaluation indices are a useful alternative. Ranking is easy if global evaluation is sufficient, but different sets of features may be important for separation of different classes, and some are important in small regions only – cf. decision trees. The lack of stability is the main problem: boosting does not help. Local selection/ranking with margins and stabilization is the most promising technique.

Open questions Discretization or kernel estimation? Best discretization: Vopt histograms, entropy, separability? Margin-based selection: like SVM. Stabilization using margins for individual vectors. Use of feature weighting from ranking/selection to scale input data. Use PCA on groups of redundant features. Evaluation indices more sensitive to local information? Statistical tests, such as Kolmogorow-Smirnov, to identify redundant features. Will anything work reliably for microarray feature selection? Different methods may use information contained in selected attributes in a different way, are filters sufficient?