Clustering Analysis With Silhouette Method

Introduction

Clustering is a popular unsupervised learning technique that involves grouping similar data points together based on their features. The Silhouette Method is a commonly used technique to determine the optimal number of clusters in a dataset. In this lab, we will use the Silhouette Method to determine the optimal number of clusters using the KMeans algorithm.

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Skills Graph

Import Libraries

We will start by importing the necessary libraries to perform the analysis.

from sklearn.datasets import make_blobs
from sklearn.cluster import KMeans
from sklearn.metrics import silhouette_samples, silhouette_score
import matplotlib.pyplot as plt
import matplotlib.cm as cm
import numpy as np

Generate Data

We will generate sample data using the make_blobs function from the sklearn.datasets library. This function generates isotropic Gaussian blobs for clustering.

X, y = make_blobs(
    n_samples=500,
    n_features=2,
    centers=4,
    cluster_std=1,
    center_box=(-10.0, 10.0),
    shuffle=True,
    random_state=1,
)  ## For reproducibility

Determine Optimal Number of Clusters

We will use the Silhouette Method to determine the optimal number of clusters for the KMeans algorithm. We will iterate through a range of values for n_clusters and plot the silhouette scores for each value.

range_n_clusters = [2, 3, 4, 5, 6]

for n_clusters in range_n_clusters:
    ## Create a subplot with 1 row and 2 columns
    fig, (ax1, ax2) = plt.subplots(1, 2)
    fig.set_size_inches(18, 7)

    ## The 1st subplot is the silhouette plot
    ax1.set_xlim([-0.1, 1])
    ax1.set_ylim([0, len(X) + (n_clusters + 1) * 10])

    ## Initialize the clusterer with n_clusters value and a random generator
    ## seed of 10 for reproducibility.
    clusterer = KMeans(n_clusters=n_clusters, n_init="auto", random_state=10)
    cluster_labels = clusterer.fit_predict(X)

    ## The silhouette_score gives the average value for all the samples.
    silhouette_avg = silhouette_score(X, cluster_labels)

    ## Compute the silhouette scores for each sample
    sample_silhouette_values = silhouette_samples(X, cluster_labels)

    y_lower = 10
    for i in range(n_clusters):
        ## Aggregate the silhouette scores for samples belonging to
        ## cluster i, and sort them
        ith_cluster_silhouette_values = sample_silhouette_values[cluster_labels == i]

        ith_cluster_silhouette_values.sort()

        size_cluster_i = ith_cluster_silhouette_values.shape[0]
        y_upper = y_lower + size_cluster_i

        color = cm.nipy_spectral(float(i) / n_clusters)
        ax1.fill_betweenx(
            np.arange(y_lower, y_upper),
            0,
            ith_cluster_silhouette_values,
            facecolor=color,
            edgecolor=color,
            alpha=0.7,
        )

        ## Label the silhouette plots with their cluster numbers at the middle
        ax1.text(-0.05, y_lower + 0.5 * size_cluster_i, str(i))

        ## Compute the new y_lower for next plot
        y_lower = y_upper + 10  ## 10 for the 0 samples

    ax1.set_title("The silhouette plot for the various clusters.")
    ax1.set_xlabel("The silhouette coefficient values")
    ax1.set_ylabel("Cluster label")

    ## The vertical line for average silhouette score of all the values
    ax1.axvline(x=silhouette_avg, color="red", linestyle="--")

    ax1.set_yticks([])  ## Clear the yaxis labels / ticks
    ax1.set_xticks([-0.1, 0, 0.2, 0.4, 0.6, 0.8, 1])

    ## 2nd Plot showing the actual clusters formed
    colors = cm.nipy_spectral(cluster_labels.astype(float) / n_clusters)
    ax2.scatter(
        X[:, 0], X[:, 1], marker=".", s=30, lw=0, alpha=0.7, c=colors, edgecolor="k"
    )

    ## Labeling the clusters
    centers = clusterer.cluster_centers_
    ## Draw white circles at cluster centers
    ax2.scatter(
        centers[:, 0],
        centers[:, 1],
        marker="o",
        c="white",
        alpha=1,
        s=200,
        edgecolor="k",
    )

    for i, c in enumerate(centers):
        ax2.scatter(c[0], c[1], marker="$%d$" % i, alpha=1, s=50, edgecolor="k")

    ax2.set_title("The visualization of the clustered data.")
    ax2.set_xlabel("Feature space for the 1st feature")
    ax2.set_ylabel("Feature space for the 2nd feature")

    plt.suptitle(
        "Silhouette analysis for KMeans clustering on sample data with n_clusters = %d"
        % n_clusters,
        fontsize=14,
        fontweight="bold",
    )

plt.show()

Interpret Results

We will interpret the results of the Silhouette Method. We will look at the average silhouette score for each value of n_clusters and choose the value that gives the highest score.

Summary

In this lab, we used the Silhouette Method to determine the optimal number of clusters for the KMeans algorithm. We generated sample data using the make_blobs function and plotted the silhouette scores for a range of values for n_clusters. We interpreted the results and chose the optimal value for n_clusters.