Unsupervised Learning and Clustering

Transcription

1 Unsupervsed Learnng and Clusterng

2 Supervsed vs. Unsupervsed Learnng Up to now we consdered supervsed learnng scenaro, where we are gven 1. samples 1,, n 2. class labels for all samples 1,, n Ths s also called learnng wth teacher, snce correct answer (the true class) s provded Today we consder unsupervsed learnng scenaro, where we are only gven 1. samples 1,, n Ths s also called learnng wthout teacher, snce correct answer s not provded do not splt data nto tranng and test sets

3 Unsupervsed Learnng Data s not labeled a lot s known easer Parametrc Approach assume parametrc dstrbuton of data estmate parameters of ths dstrbuton much harder than supervsed case Non Parametrc Approach group the data nto clusters, each cluster (hopefully) says somethng about categores (classes) present n the data lttle s known harder

4 Clusterng Seek natural clusters n the data What s a good clusterng? nternal (wthn the cluster) dstances should be small eternal (ntra-cluster) should be large Clusterng s a way to dscover new categores (classes)

5 What we Need for Clusterng 1. Promty measure, ether smlarty measure s(, k ): large f, k are smlar dssmlarty(or dstance) measure d(, k ): small f, k are smlar large d, small s large s, small d 2. Crteron functon to evaluate a clusterng good clusterng bad clusterng 3. Algorthm to compute clusterng For eample, by optmzng the crteron functon

6 How Many Clusters? 3 clusters or 2 clusters? Possble approaches 1. f the number of clusters to k 2. fnd the best clusterng accordng to the crteron functon (number of clusters may vary)

7 Promty Measures good promty measure s VERY applcaton dependent Clusters should be nvarant under the transformatons natural to the problem For eample for object recognton, should have nvarance to rotaton dstance 0 For character recognton, no nvarance to rotaton 9 6

8 Dstance (dssmlarty) Measures Manhattan (cty block) dstance ( ) ( ) ( ) = = d k k j k j d 1, Eucldean dstance ( ) ( ) ( ) ( ) = = d k k j k j d 1 2, translaton nvarant appromaton to Eucldean dstance, cheaper to compute ( ) ( ) ( ) ma, 1 k j k d k j d = Chebyshev dstance appromaton to Eucldean dstance, cheapest to compute

9 Smlarty Measures Cosne smlarty: (, ) s = j the smaller the angle, the larger the smlarty scale nvarant measure popular n tet retreval T j j Correlaton coeffcent popular n mage processng s (, ) j = d d k= 1 ( k) ( k) ( ) d ( k) 2 ( k) ( ) k= 1 k= 1 ( j) j ( j) j 2 1/ 2

10 Feature Scale old problem: how to choose approprate relatve scale for features? [length (n meters or cms?), weght(n n grams or kgs?)] In supervsed learnng, can normalze to zero mean unt varance wth no problems n clusterng ths s more problematc, f varance n data s due to cluster presence, then normalzng features s not a good thng before normalzaton after normalzaton

11 Smplest Clusterng Algorthm Havng defned a promty functon, can develop a smple clusterng algorthm go over all sample pars, and put them n the same cluster f the dstance between them s less then some threshold dstance d 0 (or f smlarty s larger than s 0 ) Pros: smple to understand and mplement Cons: very dependent on d 0 (or s 0 ), automatc choce of d 0 (or s 0 )s not an easly solved ssue d 0 too small: too many clusters d 0 larger: reasonable clusterng d 0 too large: too few clusters

12 Crteron Functons for Clusterng Have samples 1,, n Suppose parttoned samples nto c subsets D 1,,D c D 1 D 3 There are appromately c n /c! dstnct parttons D 2 Can defne a crteron functon J(D 1,,D c ) whch measures the qualty of a parttonng D 1,,D c Then the clusterng problem s a well defned problem the optmal clusterng s the partton whch optmzes the crteron functon

13 SSE Crteron Functon Let n be the number of samples n D, and defne the mean of samples n s D µ = 1 n Then the sum-of-squared errors crteron functon (to mnmze) s: c 2 JSSE = µ D = 1 D µ 1 µ 2 Note that the number of clusters, c, s fed

14 SSE Crteron Functon J SSE = c = 1 D 2 µ SSE crteron approprate when data forms compact clouds that are relatvely well separated SSE crteron favors equally szed clusters, and may not be approprate when natural groupngs have very dfferent szes large J SSE small J SSE

15 Falure Eample for J SSE larger J SSE smaller J SSE The problem s that one of the natural clusters s not compact (the outer rng)

16 Other Mnmum Varance Crteron Functons We can elmnate constant terms from c 2 JSSE = µ = 1 2 c = 1 D We get an equvalent crteron functon: J E n 1 y 2 = 1 n y D D d = average Eucldan dstance between all pars of samples n D Can obtan other crteron functons by replacng - y 2 by any other measure of dstance between ponts n D Alternatvely can replace d by the medan, mamum, etc. nstead of the average dstance 2

17 Mamum Dstance Crteron Consder J c ma = n 2 ma y = 1 y D, D Solves prevous case However J ma s not robust to outlers smallest J ma smallest J ma

18 Other Crteron Functons Recall defnton of scatter matrces scatter matr for th cluster S = ( µ )( µ ) wthn the cluster scatter matr D c S W = S Determnant of S w roughly measures the square of the volume Assumng S w s nonsngular, defne determnant crteron functon: c Jd = SW = S J d s nvarant to scalng of the as, and s useful f there are unknown rrelevant lnear transformatons of the data = 1 = 1 t

19 Iteratve Optmzaton Algorthms Now have both promty measure and crteron functon, need algorthm to fnd the optmal clusterng Ehaustve search s mpossble, snce there are appromately c n /c! possble parttons Usually some teratve algorthm s used 1. Fnd a reasonable ntal partton 2. Repeat: move samples from one group to another s.t. the objectve functon J s mproved move samples to mprove J J = 777,777 J =666,666

20 Iteratve Optmzaton Algorthms Iteratve optmzaton algorthms are smlar to gradent descent move n the drecton of descent (ascent), but not n the steepest descent drecton snce have no dervatve of the objectve functon soluton depends on the ntal pont cannot fnd global mnmum Man Issue How to move from current parttonng to the one whch mproves the objectve functon

21 K-means Clusterng We now consder an eample of teratve optmzaton algorthm for the specal case of J SSE objectve functon J SSE = k = 1 D 2 µ for a dfferent objectve functon, we need a dfferent optmzaton algorthm, of course F number of clusters to k (c = k) k-means s probably the most famous clusterng algorthm t has a smart way of movng from current parttonng to the net one

22 K-means Clusterng k = 3 1. Intalze pck k cluster centers arbtrary assgn each eample to closest center 2. compute sample means for each cluster 3. reassgn all samples to the closest mean 4. f clusters changed at step 3, go to step 2

23 K-means Clusterng Consder steps 2 and 3 of the algorthm 2. compute sample means for each cluster JSSE = µ 1 µ 2 = 1 D = sum of 3. reassgn all samples to the closest mean k µ µ 1 µ 2 by ther old means, the If we represent clusters error has gotten smaller 2

24 K-means Clusterng 3. reassgn all samples to the closest mean µ 1 µ 2 If we represent clusters by ther old means, the error has gotten smaller However we represent clusters by ther new means, and mean s always the smallest representaton of a cluster z D 1 2 z ( 2 t 2 = 2 z+ z ) 2 z 2 z = 1 n D 1 = ( + z ) D D = 0

25 K-means Clusterng We just proved that by dong steps 2 and 3, the objectve functon goes down n two step, we found a smart move whch decreases the objectve functon Thus the algorthm converges after a fnte number of teratons of steps 2 and 3 However the algorthm s not guaranteed to fnd a global mnmum µ 1 µ 2 2-means gets stuck here global mnmum of J SSE

26 K-means Clusterng Fndng the optmum of J SSE s NP-hard In practce, k-means clusterng performs usually well It s very effcent Its soluton can be used as a startng pont for other clusterng algorthms Stll 100 s of papers on varants and mprovements of k-means clusterng every year

27 Herarchcal Clusterng Up to now, consdered flat clusterng? For some data, herarchcal clusterng s more approprate than flat clusterng Herarchcal clusterng

28 Herarchcal Clusterng: Bologcal Taonomy anmal plant wth spne no spne seed producng spore producng dog cat jellyfsh apple rose mushroom mold

29 Herarchcal Clusterng: Dendogram preferred way to represent a herarchcal clusterng s a dendrogram Bnary tree Level k corresponds to parttonng wth n-k+1 clusters f need k clusters, take clusterng from level n-k+1 If samples are n the same cluster at level k, they stay n the same cluster at hgher levels dendrogram typcally shows the smlarty of grouped clusters

30 Eample

31 Herarchcal Clusterng: Venn Dagram Can also use Venn dagram to show herarchcal clusterng, but smlarty s not represented quanttatvely

32 Herarchcal Clusterng Algorthms for herarchcal clusterng can be dvded nto two types: 1. Agglomeratve (bottom up) procedures Start wth n sngleton clusters Form herarchy by mergng most smlar clusters Dvsve (top bottom) procedures Start wth all samples n one cluster Form herarchy by splttng the worst clusters

33 Dvsve Herarchcal Clusterng Any flat algorthm whch produces a fed number of clusters can be used set c = 2

34 Agglomeratve Herarchcal Clusterng ntalze wth each eample n sngleton cluster whle there s more than 1 cluster 1. fnd 2 nearest clusters 2. merge them Four common ways to measure cluster dstance 1. mnmum dstance d ( D, D ) = mn y mn j D, y D j 2. mamum dstance d (, ) ma ma D D j = y 3. average dstance 4. mean dstance D, y D j 1 ( D, D j) = d avg y n n mean j D y D j ( D D ) = µ d µ, j j

35 Sngle Lnkage or Nearest Neghbor Agglomeratve clusterng wth mnmum dstance d ( D, D ) = mn y 3 mn 5 j D, y D j generates mnmum spannng tree encourages growth of elongated clusters dsadvantage: very senstve to nose what we want at level wth c=3 what we get at level wth c=3 nosy sample

36 Complete Lnkage or Farthest Neghbor Agglomeratve clusterng wth mamum dstance d ma encourages compact clusters ( D, D ) = ma y j D, y D j Does not work well f elongated clusters present 5 ( ) D 1 ( D ) D D 2 3 d ma D1,D2 < dma 2,D3 thus D 1 and D 2 are merged nstead of D 2 and D 3

37 Average and Mean Agglomeratve Clusterng Agglomeratve clusterng s more robust under the average or the mean cluster dstance 1 ( D, D j) = d avg y n n mean j D y D j ( D D ) = µ d µ, j j mean dstance s cheaper to compute than the average dstance unfortunately, there s not much to say about agglomeratve clusterng theoretcally, but t does work reasonably well n practce

38 Agglomeratve vs. Dvsve Agglomeratve s faster to compute, n general Dvsve may be less blnd to the global structure of the data Dvsve Agglomeratve when takng the frst step (splt), have access to all the data; can fnd the best possble splt n 2 parts when takng the frst step mergng, do not consder the global structure of the data, only look at parwse structure

39 Frst (?) Applcaton of Clusterng John Snow, a London physcan plotted the locaton of cholera deaths on a map durng an outbreak n the 1850s. The locatons ndcated that cases were clustered around certan ntersectons where there were polluted wells -- thus eposng both the problem and the soluton. From: Nna Mshra HP Labs

40 Applcaton of Clusterng Astronomy SkyCat: Clustered sky objects nto stars, galaes, quasars, etc based on radaton emtted n dfferent spectrum bands. From: Nna Mshra HP Labs

41 Applcatons of Clusterng Image segmentaton Fnd nterestng objects n mages to focus attenton at From: Image Segmentaton by Nested Cuts, O. Veksler, CVPR2000

42 Applcatons of Clusterng Image Database Organzaton for effcent search

43 Applcatons of Clusterng Data Mnng Technology watch Derwent Database, contans all patents fled n the last 10 years worldwde Searchng by keywords leads to thousands of documents Fnd clusters n the database and fnd f there are any emergng technologes and what competton s up to Marketng Customer database Fnd clusters of customers and talor marketng schemes to them

44 Applcatons of Clusterng gene epresson profle clusterng smlar epressons, epect smlar functon U CL M84697 a-tub M95595 ACC X66719 ACO U41998 ACT AF ACX AF ACX U40856 AIG U40857 AIG AF AIM X92510 AOS From:De Smet F., Mathys J., Marchal K., Thjs G., De Moor B. & Moreau Y Adaptve Qualty-based clusterng of gene epresson profles, Bonformatcs, 18(6),

45 Applcatons of Clusterng Proflng Web Users Use web access logs to generate a feature vector for each user Cluster users based on ther feature vectors Identfy common goals for users Shoppng Job Seekers Product Seekers Tutorals Seekers Can use clusterng results to mprovng web content and desgn

46 Summary Clusterng (nonparametrc unsupervsed learnng) s useful for dscoverng nherent structure n data Clusterng s mmensely useful n dfferent felds Clusterng comes naturally to humans (n up to 3 dmensons), but not so to computers It s very easy to desgn a clusterng algorthm, but t s very hard to say f t does anythng good General purpose clusterng does not est, for best results, clusterng should be tuned to applcaton at hand