8 - k-means Clustering

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Questions and Answers

What is a limitation of improved algorithms in clustering?

No theoretical guarantees

What type of distances can k-means be used with?

Bregman divergences

What is the connection between cosine similarity and squared Euclidean distance?

Closely connected

What does Spherical k-means do with input data?

Normalizes it Signup and view all the answers

How does Spherical k-means minimize the average cosine similarity?

By normalizing the new centers Signup and view all the answers

Why can the solution found by the standard k-means algorithm be arbitrarily poor?

Bad starting conditions Signup and view all the answers

What is the main issue with using subtraction in computations for clustering?

Numerically unstable Signup and view all the answers

What is the importance of pairwise squared deviations in clustering?

Minimizing squared deviations from the mean Signup and view all the answers

Why is the standard algorithm for -means considered inefficient?

Little good reason to use it in practice Signup and view all the answers

What is the significance of MacQueen's algorithm in the context of clustering?

First used the name -means for a different algorithm Signup and view all the answers

How does the arithmetic mean relate to the binomial expansion in clustering?

Only holds for the arithmetic mean Signup and view all the answers

What is the main goal of the k-means clustering algorithm?

Divide data into subsets represented by their arithmetic mean to optimize the least squared error. Signup and view all the answers

Why does the k-means algorithm use squared errors instead of other distance metrics?

Squared errors put more weight on larger deviations and the arithmetic mean minimizes the squared Euclidean distance. Signup and view all the answers

What type of problem is k-means clustering considered?

Non-convex problem Signup and view all the answers

Explain why k-means clustering is suitable for signals with normally distributed measurement errors.

K-means is a good choice for signals with normal errors because it optimizes the least squared error. Signup and view all the answers

What theorem is attributed to König, Huygens, and Steiner in the context of clustering?

Steiner Translation Theorem / König-Huygens Theorem Signup and view all the answers

Why is it important to assign every point to its least-squares closest cluster in k-means clustering?

Assigning points to their least-squares closest cluster minimizes the sum of squared errors. Signup and view all the answers

Study Notes

k-Means Clustering

k-means clustering is numerically unstable in computations, but useful in proofs.
The pairwise sum of squared deviations minimizes squared deviations from the mean.

The Standard Algorithm (Lloyd's Algorithm)

The standard algorithm for k-means is not the most efficient algorithm despite being widely taught.
There are over 12 variants of the algorithm, including ELKI, which contains multiple variants.
Improved algorithms focus on reducing the number of computations for reassignment, but often lack theoretical guarantees.

k-Means for Text Clustering

k-means cannot be used with arbitrary distances, only with Bregman divergences.
Spherical k-means uses normalized input data and centers, minimizing the average cosine similarity.
Spherical k-means uses sparse nearest-centroid computations and can be accelerated using stored bounds.

Limitations of k-Means

The solution found by the standard k-means algorithm can be arbitrarily poor.
In the worst case, a k-means solution can be arbitrarily worse than the best solution.

Properties of k-Means Clustering

k-means divides data into subsets represented by their arithmetic mean.
Squared errors put more weight on larger deviations.
Arithmetic mean is the maximum likelihood estimator of centrality.
Data is split into Voronoi cells.
k-means is a non-convex problem.

The Sum of Squares Objective

The sum-of-squares objective is minimized by the arithmetic mean.
Assigning every point to its least-squares closest cluster reduces the sum of squares.
The sum of squares is equivalent to the squared Euclidean distance.

Historical Note

The history of least squares estimation is attributed to Legendre and Gauss.

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Description

Explore the concept of pairwise sum of squared deviations and the standard algorithm for K-means. Learn about the arithmetic mean and how to minimize squared deviations from the mean. Understand the implications of numerically unstable subtraction in computations.