K means and dbscan

K-MEAN CLUSTERBY CHENG ZHAN

HOUSTON MACHINE LEARNING MEETUP

1/7/2017

INTRODUCTION

• K-means (MacQueen, 1967) is one of the simplest unsupervised learning algorithms that solve the well known clustering problem.

• The main idea is to define k centroids, one for each cluster.

• Input

• M(set of points)

• k(number of clusters)

• Output

• μ_1 , …, μ_k (cluster centroids)

• k-Means clusters the M point into K clusters by minimizing the squared error function

μ

HOW TO PICK K

K-MEAN ALGORITHM

• 0. Initialize cluster centers

• 1. Assign observations to closest

cluster center

• 2. Revise cluster centers as mean of

assigned observations

• 3. Repeat 1&2 until convergence

K-MEAN ALGORITHM

• 0. Initialize cluster centers

• 1. Assign observations to closest

cluster center

• 2. Revise cluster centers as mean of

assigned observations

• 3. Repeat 1&2 until convergence

K-MEAN ALGORITHM

• 0. Initialize cluster centers

• 1. Assign observations to closest

cluster center

• 2. Revise cluster centers as mean of

assigned observations

• 3. Repeat 1&2 until convergence

K-MEAN ALGORITHM

• 0. Initialize cluster centers

• 1. Assign observations to closest

cluster center

• 2. Revise cluster centers as mean of

assigned observations

• 3. Repeat 1&2 until convergence

GOOD INITIAL POINTS

UNLUCKY

K-MEANS IN PRACTICE

• How to choose initial centroids

• select randomly among the data points

• generate completely randomly

• How to choose k

• study the data

• run k-Means for different k (measure squared error for each k)

• Run k-means many times!

• Get many choices of initial points

WHAT ABOUT THIS?

QUESTIONS

• Euclidean distance results in spherical clusters

• What cluster shape does the Manhattan distance give?

• Think of other distance measures. What cluster shapes will those yield?

DBSCAN

DENSITY-BASED SPATIAL CLUSTERING OF APPLICATIONWITH NOISE

• DBSCAN is a Density-Based Clustering algorithm

• In density based clustering we partition points into dense regions separated by not-so-dense regions.

• Important Questions:

• How do we measure density and what is a dense region?

• DBSCAN:

• Density at point p: number of points within a circle of radius Eps

• Dense Region: A circle of radius Eps that contains at least MinPts points

WHEN DBSCAN WORKS WELL

WHEN DBSCAN WORKS WELL

DBSCAN

DEFINITIONS

REACHABILITY AND CONNECTIVITY

DBSCAN ALGORITHM: EXAMPLE

DBSCAN ALGORITHM: EXAMPLE

DBSCAN ALGORITHM: EXAMPLE

DETERMINING EPS & MINPTS

• Idea is that for points in a cluster, their kth nearest neighbors are at roughly the same distance

• Noise points have the kth nearest neighbor at farther distance

• So, plot sorted distance of every point to its kth nearest neighbor

• Find the distance d where there is a “knee” in the curve

• Eps = d, MinPts = k

SENSITIVE TO PARAMETERS

SENSITIVE TO PARAMETERS

APPLICATIONS

DISTANCE METRIC FOR DOCUMENTS

• Motivations

• Identical – easy

• Modified or related (Ex: DNA, Plagiarism, Authorship)

• Did Francis Bacon write Shakespeare’s plays

DOCUMENT RETRIEVAL

CHALLENGES

• How do we measure similarity

• How do we search over articles

DOCUMENT REPRESENTATION

• Word count document representation

• Bag of words model

• Ignore order of words

• Count # of instances of each word in vocabulary

EXAMPLE

• Word: Sequence of alphanumeric characters. For example, the phrase “6.006 is fun” has 4 words.

• Word Frequencies: Word frequency D(w) of a given word w is the number of times it occurs in a document D.

• For example, the words and word frequencies for the above phrase are as below:

Word 6 The Is 006 Easy Fun

Count 1 0 1 1 0 1

METRIC

• d(x,x) = 0

• d(x,y) = d(y,x)

• d(x,y) + d(y,z) >= d(x,z)

METRIC

• Inner product of the vectors D1 andD2 containing the word frequencies for all words in the 2 documents. Equivalently, this is the projection of vectors D1 onto D2 or vice versa. Mathematically this is expressed as:

D1 ·D2 = ∑ D1(w) .D2(w)

• Angle Metric: The angle between the vectors D1 and D2 gives an indication of overlap between the 2 documents. Mathematically this angle is expressed as:

θ(D1,D2) = arccos (𝐷1.𝐷2

| 𝐷1 |∗| 𝐷2 |)

PYTHON EXAMPLE

• https://ocw.mit.edu/ans7870/6/6.006/s08/lecturenotes/files/docdist2.py

REFERENCE

• http://www.cs.haifa.ac.il/~rita/uml_course/lectures/kmeans.pdf

• https://cs.wmich.edu/alfuqaha/summer14/cs6530/lectures/ClusteringAnalysis.pdf

• http://www.it.uu.se/edu/course/homepage/infoutv/ht09/a2t.pdf

• http://www.cse.buffalo.edu/~jing/cse601/fa12/materials/clustering_density.pdf

• Machine Learning Specialization by University of Washington in Coursera