1 Clustering Preliminaries Applications Euclidean/Non-Euclidean Spaces Distance Measures

2 The Problem of Clustering uGiven a set of points, with a notion of distance between points, group the points into some number of clusters, so that members of a cluster are in some sense as close to each other as possible.

3 Example x x x x x x x xx x x x x x x x x x x x x x x x x x x

4 Problems With Clustering uClustering in two dimensions looks easy. uClustering small amounts of data looks easy. uAnd in most cases, looks are not deceiving.

5 The Curse of Dimensionality uMany applications involve not 2, but 10 or 10,000 dimensions. uHigh-dimensional spaces look different: almost all pairs of points are at about the same distance.

6 Example: Curse of Dimensionality uAssume random points within a bounding box, e.g., values between 0 and 1 in each dimension. uIn 2 dimensions: a variety of distances between 0 and uIn 10,000 dimensions, the difference in any one dimension is distributed as a triangle.

7 Example – Continued uThe law of large numbers applies. uActual distance between two random points is the sqrt of the sum of squares of essentially the same set of differences.

8 Example High-Dimension Application: SkyCat uA catalog of 2 billion “sky objects” represents objects by their radiation in 7 dimensions (frequency bands). uProblem: cluster into similar objects, e.g., galaxies, nearby stars, quasars, etc. uSloan Sky Survey is a newer, better version.

9 Example: Clustering CD’s (Collaborative Filtering) uIntuitively: music divides into categories, and customers prefer a few categories. wBut what are categories really? uRepresent a CD by the customers who bought it. uSimilar CD’s have similar sets of customers, and vice-versa.

10 The Space of CD’s uThink of a space with one dimension for each customer. wValues in a dimension may be 0 or 1 only. uA CD’s point in this space is (x 1, x 2,…, x k ), where x i = 1 iff the i th customer bought the CD. wCompare with boolean matrix: rows = customers; cols. = CD’s.

11 Space of CD’s – (2) uFor Amazon, the dimension count is tens of millions. uAn alternative: use minhashing/LSH to get Jaccard similarity between “close” CD’s. u1 minus Jaccard similarity can serve as a (non-Euclidean) distance.

12 Example: Clustering Documents uRepresent a document by a vector (x 1, x 2,…, x k ), where x i = 1 iff the i th word (in some order) appears in the document. wIt actually doesn’t matter if k is infinite; i.e., we don’t limit the set of words. uDocuments with similar sets of words may be about the same topic.

13 Aside: Cosine, Jaccard, and Euclidean Distances uAs with CD’s we have a choice when we think of documents as sets of words or shingles: 1.Sets as vectors: measure similarity by the cosine distance. 2.Sets as sets: measure similarity by the Jaccard distance. 3.Sets as points: measure similarity by Euclidean distance.

14 Example: DNA Sequences uObjects are sequences of {C,A,T,G}. uDistance between sequences is edit distance, the minimum number of inserts and deletes needed to turn one into the other. uNote there is a “distance,” but no convenient space in which points “live.”

15 Distance Measures uEach clustering problem is based on some kind of “distance” between points. uTwo major classes of distance measure: 1.Euclidean 2.Non-Euclidean

16 Euclidean Vs. Non-Euclidean uA Euclidean space has some number of real-valued dimensions and “dense” points. wThere is a notion of “average” of two points. wA Euclidean distance is based on the locations of points in such a space. uA Non-Euclidean distance is based on properties of points, but not their “location” in a space.

17 Axioms of a Distance Measure ud is a distance measure if it is a function from pairs of points to real numbers such that: 1.d(x,y) > 0. 2.d(x,y) = 0 iff x = y. 3.d(x,y) = d(y,x). 4.d(x,y) < d(x,z) + d(z,y) (triangle inequality ).

18 Some Euclidean Distances uL 2 norm : d(x,y) = square root of the sum of the squares of the differences between x and y in each dimension. wThe most common notion of “distance.” uL 1 norm : sum of the differences in each dimension. wManhattan distance = distance if you had to travel along coordinates only.

19 Examples of Euclidean Distances x = (5,5) y = (9,8) L 2 -norm: dist(x,y) =  ( ) = 5 L 1 -norm: dist(x,y) = 4+3 =

20 Another Euclidean Distance  L ∞ norm : d(x,y) = the maximum of the differences between x and y in any dimension.  Note: the maximum is the limit as n goes to ∞ of what you get by taking the n th power of the differences, summing and taking the n th root.

21 Non-Euclidean Distances uJaccard distance for sets = 1 minus ratio of sizes of intersection and union. uCosine distance = angle between vectors from the origin to the points in question. uEdit distance = number of inserts and deletes to change one string into another.

22 Jaccard Distance for Sets (Bit-Vectors) uExample: p 1 = 10111; p 2 = uSize of intersection = 3; size of union = 4, Jaccard similarity (not distance) = 3/4. ud(x,y) = 1 – (Jaccard similarity) = 1/4.

23 Why J.D. Is a Distance Measure ud(x,x) = 0 because x  x = x  x. ud(x,y) = d(y,x) because union and intersection are symmetric. ud(x,y) > 0 because |x  y| < |x  y|. ud(x,y) < d(x,z) + d(z,y) trickier – next slide.

24 Triangle Inequality for J.D. 1 - |x  z| |y  z| > 1 -|x  y| |x  z| |y  z| |x  y| uRemember: |a  b|/|a  b| = probability that minhash(a) = minhash(b). uThus, 1 - |a  b|/|a  b| = probability that minhash(a)  minhash(b).

25 Triangle Inequality – (2) uClaim: prob[minhash(x)  minhash(y)] < prob[minhash(x)  minhash(z)] + prob[minhash(z)  minhash(y)] uProof: whenever minhash(x)  minhash(y), at least one of minhash(x)  minhash(z) and minhash(z)  minhash(y) must be true.

26 Similar Sets and Clustering uWe can use minhashing + LSH to find quickly those pairs of sets with low Jaccard distance. uWe can cluster sets (points) using J.D. uBut we only know some distances – the low ones. uThus, clusters are not always connected components.

27 Example: Clustering + J.D. {a,b,c} {b,c,e,f} {d,e,f} {a,b,d,e} Similarity threshold = 1/3; distance < 2/3

28 Cosine Distance uThink of a point as a vector from the origin (0,0,…,0) to its location. uTwo points’ vectors make an angle, whose cosine is the normalized dot- product of the vectors: p 1.p 2 /|p 2 ||p 1 |. wExample: p 1 = 00111; p 2 = wp 1.p 2 = 2; |p 1 | = |p 2 | =  3.  cos(  ) = 2/3;  is about 48 degrees.

29 Cosine-Measure Diagram p1p1 p2p2 p 1.p 2  |p 2 | d (p 1, p 2 ) =  = arccos(p 1.p 2 /|p 2 ||p 1 |)

30 Why C.D. Is a Distance Measure ud(x,x) = 0 because arccos(1) = 0. ud(x,y) = d(y,x) by symmetry. ud(x,y) > 0 because angles are chosen to be in the range 0 to 180 degrees. uTriangle inequality: physical reasoning. If I rotate an angle from x to z and then from z to y, I can’t rotate less than from x to y.

31 Edit Distance uThe edit distance of two strings is the number of inserts and deletes of characters needed to turn one into the other. Equivalently: u d(x,y) = |x| + |y| - 2|LCS(x,y)|. wLCS = longest common subsequence = any longest string obtained both by deleting from x and deleting from y.

32 Example: LCS ux = abcde ; y = bcduve. uTurn x into y by deleting a, then inserting u and v after d. wEdit distance = 3. uOr, LCS(x,y) = bcde. uNote: |x| + |y| - 2|LCS(x,y)| = –2*4 = 3 = edit distance.

33 Why Edit Distance Is a Distance Measure ud(x,x) = 0 because 0 edits suffice. ud(x,y) = d(y,x) because insert/delete are inverses of each other. ud(x,y) > 0: no notion of negative edits. uTriangle inequality: changing x to z and then to y is one way to change x to y.

34 Variant Edit Distances uAllow insert, delete, and mutate. wChange one character into another. uMinimum number of inserts, deletes, and mutates also forms a distance measure. uDitto for any set of operations on strings. wExample: substring reversal OK for DNA sequences