CS434a/541a: Pattern Recognition Prof. Olga Veksler. Lecture 15

Transcription

1 CS434a/541a: Pattern Recognton Prof. Olga Veksler Lecture 15

2 Today New Topc: Unsupervsed Learnng Supervsed vs. unsupervsed learnng Unsupervsed learnng Net Tme: parametrc unsupervsed learnng Today: nonparametrc unsupervsed learnng = clusterng Promty Measures Crteron Functons Flat Clusterng k-means Herarchcal Clusterng Dvsve Agglomeratve

3 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 In the net few lectures 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

4 Unsupervsed Learnng Data s not labeled a lot s known easer 1. Parametrc Approach assume parametrc dstrbuton of data estmate parameters of ths dstrbuton much harder than supervsed case NonParametrc Approach group the data nto clusters, each cluster (hopefully) says somethng about categores (classes) present n the data lttle s known harder

5 Why Unsupervsed Learnng? Unsupervsed learnng s harder How do we know f results are meanngful? No answer labels are avalable. Let the epert look at the results (eternal evaluaton) Defne an objectve functon on clusterng (nternal evaluaton) We nevertheless need t because 1. Labelng large datasets s very costly (speech recognton) sometmes can label only a few eamples by hand 2. May have no dea what/how many classes there are (data mnng) 3. May want to use clusterng to gan some nsght nto the structure of the data before desgnng a classfer Clusterng as data descrpton

6 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)

7 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

8 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)

9 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 large dstance

10 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, 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 translaton nvarant

11 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 2 1/ 2 large smlarty small smlarty

12 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

13 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

14 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

15 SSE Crteron Functon Let n be the number of samples n D, and defne the mean of samples n n 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

16 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

17 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)

18 Other Mnmum Varance Crteron Functons We can rewrte the SSE as J SSE = c = 1 D µ 2 = 1 2 c 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

19 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

20 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

21 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

22 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

23 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

24 K-means Clusterng 1. Intalze pck k cluster centers arbtrary assgn each eample to closest center k = 3 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

25 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 3. reassgn all samples to the closest mean k = sum of µ µ 1 µ 2 by ther old means, the error has gotten smaller If we represent clusters 2

26 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

27 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

28 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

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

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

31 Herarchcal Clusterng: Dendogram prefered way to represent a herarchcal clusterng s a dendogram 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 dendogram typcally shows the smlarty of grouped clusters

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

33 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

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

35 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

36 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 what we get nosy sample

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

38 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

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

40 Summary Clusterng (nonparametrc 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 s unlkely to est, for best results, clusterng should be tuned to applcaton at hand