Overview

Teaching: 15 min
Exercises: 20 min

Questions

• How can we use clustering to find data points with similar attributes?

Objectives

• Identify clusters in data using k-means clustering.

• See the limitations of k-means when clusters overlap.

• Use spectral clustering to overcome the limitations of k-means.

Clustering

Clustering is the grouping of data points which are similar to each other. It can be a powerful technique for identifying patterns in data. Clustering analysis does not usually require any training and is known as an unsupervised learning technique. The lack of a need for training means it can be applied quickly.

Applications of Clustering

• Looking for trends in data
• Data compression, all data clustering around a point can be reduced to just that point. For example reducing colour depth of an image.
• Pattern recognition

K-means Clustering

The K-means clustering algorithm is a simple clustering algorithm that tries to identify the centre of each cluster. It does this by searching for a point which minimises the distance between the centre and all the points in the cluster. The algorithm needs to be told how many clusters to look for, but a common technique is to try different numbers of clusters and combine it with other tests to decide on the best combination.

K-means with Scikit Learn

To perform a k-means clustering with Scikit learn we first need to import the sklearn.cluster module.

import sklearn.cluster as skl_cluster

For this example we’re going to use scikit learn’s built in random data blob generator instead of using an external dataset. For this we’ll also need the sklearn.datasets.samples_generator module.

import sklearn.datasets.samples_generator as skl_smpl

Now lets create some random blobs using the make_blobs function. The n_samples argument sets how many points we want to use in all of our blobs. cluster_std sets the standard deviation of the points, the smaller this value the closer together they will be. centers sets how many clusters we’d like. random_state is the initial state of the random number generator, by specifying this we’ll get the same results every time we run the program. If we don’t specify a random state then we’ll get different points every time we run. This function returns two things, an array of data points and a list of which cluster each point belongs to.

data, cluster_id = skl_smpl.make_blobs(n_samples=400, cluster_std=0.75, centers=4, random_state=1)

Now that we have some data we can go ahead and try to identify the clusters using K-means. First we need to initalise the KMeans module and tell it how many clusters to look for. Next we supply it some data via the fit function, in much the same we did with the regression functions earlier on. Finally we run the predict function to find the clusters.

Kmean = skl_cluster.KMeans(n_clusters=4)
Kmean.fit(data)
clusters = Kmean.predict(data)

The data can now be plotted to show all the points we randomly generated. To make it clearer which cluster points have been classified to we can set the colours (the c parameter) to use the clusters list that was returned by the predict function. The Kmeans algorithm also lets us know where it identified the centre of each cluster as. These are stored as a list called cluster_centers_ inside the Kmean object. Lets go ahead and plot the points from the clusters, colouring them by the output from the K-means algorithm and also plot the centres of each cluster as a red X.

plt.scatter(data[:, 0], data[:, 1], s=5, linewidth=0, c=clusters)
for cluster_x, cluster_y in Kmean.cluster_centers_:
    plt.scatter(cluster_x, cluster_y, s=100, c='r', marker='x')

import sklearn.cluster as skl_cluster
import sklearn.datasets.samples_generator as skl_smpl
import matplotlib as plt

data, cluster_id = skl_smpl.make_blobs(n_samples=400, cluster_std=0.75, centers=4, random_state=1)

Kmean = skl_cluster.KMeans(n_clusters=4)
Kmean.fit(data)
clusters = Kmean.predict(data)

plt.scatter(data[:, 0], data[:, 1], s=5, linewidth=0, c=clusters)
for cluster_x, cluster_y in Kmean.cluster_centers_:
    plt.scatter(cluster_x, cluster_y, s=100, c='r', marker='x')

Working in multiple dimensions

Although this example shows two dimensions the kmeans algorithm can work in more than two, it just becomes very difficult to show this visually once we get beyond 3 dimensions. Its very common in machine learning to be working with multiple variables and so our classifiers are working in multi-dimensonal spaces.

Limitations of K-Means

• Requires number of clusters to be known in advance
• Struggles when clusters have irregular shapes
• Will always produce an answer finding the required number of clusters even if the data isn’t clustered (or clustered in that many clusters).
• Requires linear cluster boundaries

Advantages of K-Means

• Simple algorithm, fast to compute. A good choice as the first thing to try when attempting to cluster data.
• Suitable for large datasets due to its low memory and computing requirements.

Excercise: K-Means with overlapping clusters

Adjust the program above to increase the standard deviation of the blobs (the cluster_std parameter to make_blobs) and increase the number of samples (n_samples) to 4000. You should start to see the clusters overlapping. Do the clusters that are identified make sense? Is there any strange behaviour from this?

Solution

The resulting image from increasing n_samples to 4000 and cluster_std to 3.0 looks like this: The straight line boundaries between clusters look a bit strange.

Exercise: How many clusters should we look for?

As K-Means requires us to specify the number of clusters to expect a common strategy to get around this is to vary the number of clusters we are looking for. Modify the program to loop through searching for between 2 and 10 clusters. Which (if any) of the results look more sensible? What criteria might you use to select the best one?

Solution

    for cluster_count in range(2,11):
        Kmean = skl_cluster.KMeans(n_clusters=cluster_count)
        Kmean.fit(data)
        clusters = Kmean.predict(data)
        plt.scatter(data[:, 0], data[:, 1], s=5, linewidth=0,c=clusters)
        for cluster_x, cluster_y in Kmean.cluster_centers_:
           plt.scatter(cluster_x, cluster_y, s=100, c='r', marker='x')
           # give the graph a title with the number of clusters
           plt.title(str(cluster_count)+" Clusters")
           plt.show()

None of these look very sensible clusterings because all the points really form one large cluster. We might look at a measure of similarity of the cluster to test if its really multiple clusters. A simple standard deviation or interquartile range might be a good starting point.

Spectral Clustering

Spectral clustering is a technique that attempts to overcome the linear boundary problem of k-means clustering. It works by treating clustering as a graph partitioning problem, its looking for nodes in a graph with a small distance between them. See this introduction to Spectral Clustering if you are interested in more details about how spectral clustering works.

Here is an example of using spectral clustering on two concentric circles

Spectral clustering uses something called a kernel trick to introduce additional dimensions to the data. A common example of this is trying to cluster one circle within another (concentric circles). A K-means classifier will fail to do this and will end up effectively drawing a line which crosses the circles.
Spectral clustering will introduce an additional dimension that effectively moves one of the circles away from the other in the additional dimension. This has the downside of being more computationally expensive than k-means clustering.

Spectral Clustering with Scikit Learn

Lets try out using Scikit Learn’s spectral clustering. To make the concentric circles in the above example we need to use the make_circles function in the sklearn.datasets module. This works in a very similar way to the make_blobs function we used earlier on.

import sklearn.datasets as skl_data

circles, circles_clusters = skl_data.make_circles(n_samples=400, noise=.01, random_state=0)

The code for calculating the SpectralClustering is very similar to the kmeans clustering, instead of using the sklearn.cluster.KMeans class we use the sklearn.cluster.SpectralClustering class.

model = skl_cluster.SpectralClustering(n_clusters=2, affinity='nearest_neighbors', assign_labels='kmeans')

The SpectralClustering class combines the fit and predict functions into a single function called fit_predict.

labels = model.fit_predict(circles)

Here is the whole program combined with the kmeans clustering for comparison. Note that this produces two figures, to view both of them use the “Inline” graphics terminal inside the Python console instead of the “Automatic” method which will open a window and only show you one of the graphs.

import sklearn.cluster as skl_cluster
import sklearn.datasets as skl_data

circles, circles_clusters = skl_data.make_circles(n_samples=400, noise=.01, random_state=0)

# cluster with kmeans
Kmean = skl_cluster.KMeans(n_clusters=2)
Kmean.fit(circles)
clusters = Kmean.predict(circles)

# plot the data, colouring it by cluster
plt.scatter(circles[:, 0], circles[:, 1], s=15, linewidth=0.1, c=clusters,cmap='flag')
plt.show()

# cluster with spectral clustering
model = skl_cluster.SpectralClustering(n_clusters=2, affinity='nearest_neighbors', assign_labels='kmeans')
labels = model.fit_predict(circles)
plt.scatter(circles[:, 0], circles[:, 1], s=15, linewidth=0, c=labels, cmap='flag')
plt.show()

Comparing k-means and spectral clustering performance

Modify the program we wrote in the previous exercise to use spectral clustering instead of k-means, save it as a new file. Time how long both programs take to run. Add the line import time at the top of both files, as the first line in the file get the start time with start_time = time.time(). End the program by getting the time again and subtracting the start time from it to get the total run time. Add end_time = time.time() and print("Elapsed time:",end_time-start_time,"seconds") to the end of both files. Compare how long both programs take to run generating 4,000 samples and testing them for between 2 and 10 clusters. How much did your run times differ? How much do they differ if you increase the number of samples to 8,000? How long do you think it would take to compute 800,000 samples (estimate this, it might take a while to run for real)?

Solution

KMeans version, runtime around 4 seconds (your computer might be faster/slower)

import matplotlib.pyplot as plt
import sklearn.cluster as skl_cluster
import sklearn.datasets.samples_generator as skl_smpl
import time

start_time = time.time()
data, cluster_id = skl_smpl.make_blobs(n_samples=4000, cluster_std=3,
                                       centers=4, random_state=1)

for cluster_count in range(2,11):
    Kmean = skl_cluster.KMeans(n_clusters=cluster_count)
    Kmean.fit(data)
    clusters = Kmean.predict(data)

    plt.scatter(data[:, 0], data[:, 1], s=15, linewidth=0, c=clusters)
    plt.title(str(cluster_count)+" Clusters")
    plt.show()

end_time = time.time()
print("Elapsed time = ", end_time-start_time, "seconds")

Spectral version, runtime around 9 seconds (your computer might be faster/slower)

import matplotlib.pyplot as plt
import sklearn.cluster as skl_cluster
import sklearn.datasets.samples_generator as skl_smpl
import time
start_time = time.time()
data, cluster_id = skl_smpl.make_blobs(n_samples=4000, cluster_std=3,
                                       centers=4, random_state=1)

for cluster_count in range(2,11):
    model = skl_cluster.SpectralClustering(n_clusters=cluster_count,
                                       affinity='nearest_neighbors',
                                       assign_labels='kmeans')
    labels = model.fit_predict(data)
    plt.scatter(data[:, 0], data[:, 1], s=15, linewidth=0, c=labels)
    plt.title(str(cluster_count)+" Clusters")
    plt.show()
end_time = time.time()
print("Elapsed time = ", end_time-start_time, "seconds")

When the number of points increases to 8000 the runtimes are 24 seconds for the spectral version and 5.6 seconds for kmeans. The runtime numbers will differ depending on the speed of your computer, but the relative different should be similar. For 4000 points kmeans took 4 seconds, spectral 9 seconds, 2.25 fold difference. For 8000 points kmeans took 5.6 seconds, spectral took 24 seconds. 4.28 fold difference. Kmeans 1.4 times slower for double the data, spectral 2.6 times slower. The realative difference is diverging. Its double by doubling the amount of data. If we use 100 times more data we might expect a 100 fold divergence in execution times. Kmeans might take a few minutes, spectral will take hours.

Key Points

• Clustering is a form of unsupervised learning

• Unsupervised learning algorithms don’t need training

• Kmeans is a popular clustering algorithm.

• Kmeans struggles where one cluster exists within another, such as concentric circles.

• Spectral clustering is another technique which can overcome some of the limitations of Kmeans.

• Spectral clustering is much slower than Kmeans.

• As well as providing machine learning algorithms scikit learn also has functions to make example data