Clustering VOutline• Validating clustering results• Randomization testsCluster Validity • All clustering algorithms provided with a set of points output a clustering• How to evaluate the “goodness” of the resulting clusters?• Tricky because “clusters are in the eye of the beholder”! • Then why do we want to evaluate them?– To compare clustering algorithms– To compare two sets of clusters– To compare two clusters– To decide whether there is noise in the dataClusters found in Random Data0 0.2 0.4 0.6 0.8 100.10.20.30.40.50.60.70.80.91xyRandom Points0 0.2 0.4 0.6 0.8 100.10.20.30.40.50.60.70.80.91xyK-means0 0.2 0.4 0.6 0.8 100.10.20.30.40.50.60.70.80.91xyDBSCAN0 0.2 0.4 0.6 0.8 100.10.20.30.40.50.60.70.80.91xyComplete LinkUse the objective function F• Dataset X, Objective function F• Algorithms: A1, A2,…Ak• Question: Which algorithm is the best for this objective function?• R1= A1(X), R2= A2(X),…,Rk=Ak(X)• Compare F(R1), F(R2),…,F(Rk)Evaluating clusters• Function H computes the cohesiveness of a cluster (e.g., smaller values larger cohesiveness)• Examples of cohesiveness?• Goodness of a cluster c is H(c)• c is better than c’ if H(c) < H(c’)Evaluating clusterings using cluster cohesiveness?• For a clustering C consisting of k clusters c1,…,ck• H(C) = ΦiH(ci)• What is Φ ?Cluster separation?• Function S that measures the separation between two clusters ci, cj• Ideas for S(ci,cj)? • How can we measure the goodness of a clustering C = {c1,…,ck} using the separation function S?• Silhouette Coefficient combines ideas of both cohesion and separation, but for individual points, as well as clusters and clusterings• For an individual point, I– a = average distance of i to the points in the same cluster– b = min (average distance of i to points in another cluster)– silhouette coefficient of i: s = 1 – a/b if a < b– Typically between 0 and 1. – The closer to 1 the better.• Can calculate the Average Silhouette width for a cluster or a clusteringSilhouette Coefficientab“The validation of clustering structures is the most difficult and frustrating part of cluster analysis. Without a strong effort in this direction, cluster analysis will remain a black art accessible only to those true believers who have experience and great courage.”Algorithms for Clustering Data, Jain and DubesFinal Comment on Cluster ValidityAssessing the significance of clustering (and other data mining) results• Dataset X and algorithm A• Beautiful result A(D)• But: what does it mean?• How to determine whether the result is really interesting or just due to chance?Examples• Pattern discovery: frequent itemsets or association rules• From data X we can find a collection of nice patterns• Significance of individual patterns is sometimes straightforward to test• What about the whole collection of patterns? Is it surprising to see such a collection?Examples• In clustering or mixture modeling: we always get a result• How to test if the whole idea of components/clusters in the data is good?• Do they really exist clusters in the data?Classical methods – Hypothesis testing• Example: Two datasets of real numbers X and Y (|X|=|Y|=n)• Question: Are the means of X and Y (resp. E(X), E(Y)) are significantly different• Test statistic: t = (E(X) – E(Y))/s, (s: an estimate of the standard deviation)• The test statistic follows (under certain assumptions) the t distribution with 2n-2 degrees of freedomClassical methods – Hypothesis testing• The result can be something like: “the difference in the means is significant at the level of 0.01”• That is, if we take two samples of size n, such a difference would occur by chance only in about 1 out of 100 trials• Problems:– What if we are testing many hypotheses (multiple hypotheses testing)– What if there is no closed form available?Classical methods: testing independence• Are columns X and Y independent?• Independence: Pr(X,Y) = Pr(X)*Pr(Y)• Pr(X=1) = 8/11, Pr(X=0)=3/11, Pr(Y=1) = 8/11, Pr(Y=0) = 3/11• Actual joint probabilities: Pr(X=1,Y=1) = 6/11, Pr(X=1,Y=0)=2/11, Pr(X=0,Y=1) = 2/11, Pr(X=0.Y=0)=1/11• Expected joint probabilities: Pr(X=1,Y=1) = 64/121, Pr(X=1,Y=0)=24/121, Pr(X=0,Y=1) = 24/121, Pr(X=0,Y=0)=9/121Testing independence using χ2• Are columns X and Y independent?Y=1Y=0∑rowX=1628Y=0213∑column83112}1,0{},{22],[),(],[yxyYxXEyYxXOyYxXE• So what?Classical methods – Hypothesis testing• The result can be something like: “the independence between X and Y is significant at the level of 0.01”• That is, if we take two columns X and Y with the observed P(X=1) and P(Y=1) and n rows, such degree of independence would occur by chance only in about 1 out of 100 trialsProblems with classical methods• What if we are testing many hypotheses (multiple hypotheses testing)• What if there is no closed form available?Randomization methods• Goal: assessing the significance of results– Could the result have occurred by chance?• Methodology: create datasets that somehow reflect the characteristics of the true dataRandomization methods• Create randomized versions from the data X• X1, X2,…,Xk• Run algorithm A on these, producing results A(X1), A(X2),…,A(Xk)• Check if the result A(X) on the real data is somehow different from these• Empirical p-value: the fraction of cases for which the result on real data is (say) larger than A(X)• If the empirical p-value is small, then there is something interesting in the dataRandomization for testing independence• Px= Pr(X=1) and Py= Pr(Y=1)• Generate random instances of columns (Xi,Yi) with parameters Pxand Py[independence assumption]• p-value: Compute the in how many random instances, the χ2statistic is greater/smaller than its value in the input dataRandomization methods for other tasks• Instantiation of randomization for clustering?• Instantiation of randomization for frequent-itemset miningColumnwise randomization: no global view of the dataColumnwise randomization: no global view of the dataX and Y are not more surprisingly correlated given that they both have 1s in dense rows and 0s in sparse rowsQuestions• What is a good way of randomizing the data?• Can the sample X1, X2, …, Xkbe computed efficiently?• Can the values A(X1), A(X2), …, A(Xk) be computed efficiently?What is a good way of randomizing the data?• How are datasets Xigenerated?• What
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