#10601

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[rameshoswal]

K means Clustering - 10601

Clustering:

Fully Unsupervisd Algo

Automatically partition unlabeled data points  into groups of data points

Useful for

              Organizin data

              Understanding Hidden Structure

 Appliations:

              Cluster NewsArticles, Web Pages or search results by topic.

              Cluster Protein sequences by func or genes according to expression profile.

              Cluster user of Social network based on intrest(Commuity detection) - Ad target

Optimization Background:

              Coordinate Descent:

                             Goal - Minimize a function J(theta)

                                            eg. theta = argmin(J(theta))

                             Idea: Pick one dimension, and minimize along that dimension.

                                                                Algorithm:          

1.      Choosing Initial Point heta

2.      Repeat until stopping criteria is reached

a.      Theta1 = argmin J(theta1,theta2,….thetan)

b.      Theta2 = argmin J(theta1,theta2,….thetan)

c.      Theta1 = argmin J(theta1,theta2,….thetan)

.

.

d.      Thetan = argmin J(theta1,theta2,….thetan)

Note: Steps abc..,d are exact line search alo ng same axis.

                              In some cases this algo can get stuck and start oscillating along two points. In this scenario we can use something called as Block Coordinate Descent.

              Block Coordinate Descent:

                                                           Here:  An eg. With two block alpha and beta where theta = [alpha, beta]

                                                           Goal: alpha,beta = argmin J(alpha,beta) where alpha in Ra and beta in Rb

                                                                               Idea: Minimize over an entire group of variables at a time.

                                                           Algo:

1.)    Choose alpha and beta

2.)    Repeat until stopping criteria is reached

a.      Alpha – argmin (J(alpha,beta))

b.      Beta – argmin (J(alpha,beta))              

   Clustering:

Left figure - Distance between two points in same cluster is less than different cluster

Right Figure - Distance between two points in same cluster is more than different cluster

             Now we define a Object function to minimize.

What is clustering? Goal is to partition unlabeled instances into group of similar points.

Input: Unlabeld data D = {X1,X2,X3….Xn}, X(i) belongs Rm

              *We do not know the labels!

Output:

View#1: Labeled Instances {(X(1),Z(1)), (X(2),Z(2)), (X(3),Z(3))….. (X(n),Z(n))}

                                                                                         Where Z(1) E {1,2,3,….,K} k is # of clusters

View#2: Clusterings: C1,C2,…..Ck where k is number of clusters Ci = {X(i): Z(i)=j} pints in jth partition

Questions:

1.)    How many clusters are there?

2.)    How do we define similarity between points? Eg. Eucledian distance

Object based clusteting:

              Eg K-Means Objective -    

                             Input: D = {Xi}i=1N

                             Cluster Centers: {c1,c2,…,ck} = c

Decision Rule: Assign point X(i) to its nearest cluster center cj

Objective:

              C= argmin sumi=1N minj in {1,..,k}||X(i)-cj||2

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