CS 312 AI Clustering Algorithms
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Questions and Answers

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

  • To equalize similarity across all clusters
  • To minimize within-cluster variances (correct)
  • To increase the number of clusters
  • To maximize within-cluster variances
  • What does the term 'centroid' refer to in k-means clustering?

  • A process for selecting the number of clusters
  • The average distance of data points in a cluster
  • A data point that represents the cluster's center (correct)
  • The sum of all Euclidean distances within a cluster
  • Which step in the k-means algorithm involves assigning data points to clusters?

  • Centroid selection step
  • Expectation step (correct)
  • Minimization step
  • Maximization step
  • What method is used to evaluate the quality of cluster assignments in k-means clustering?

    <p>Sum of squared errors (SSE) (D)</p> Signup and view all the answers

    What characteristic is NOT desired in k-means clustering between different clusters?

    <p>High similarity between clusters (B)</p> Signup and view all the answers

    How is the new centroid calculated in the k-means algorithm?

    <p>By computing the mean of all the points for each cluster (C)</p> Signup and view all the answers

    What does the value of k represent in k-means clustering?

    <p>The number of clusters to be formed (D)</p> Signup and view all the answers

    What is the significance of the initialization of centroids in the k-means algorithm?

    <p>It affects the final clustering result and computational efficiency (A)</p> Signup and view all the answers

    What does the Elbow method help determine when choosing the number of clusters?

    <p>The optimal number of clusters based on SSE (A)</p> Signup and view all the answers

    What happens to the SSE as more clusters are added using the Elbow method?

    <p>SSE decreases as k increases (D)</p> Signup and view all the answers

    What does the silhouette coefficient measure in clustering?

    <p>The similarity of data points within a cluster (A)</p> Signup and view all the answers

    What range of values can the silhouette coefficient take?

    <p>-1 to 1 (C)</p> Signup and view all the answers

    What significance does the elbow point have in the Elbow method?

    <p>It represents a good trade-off between error and number of clusters (D)</p> Signup and view all the answers

    Which method evaluates how well a data point fits into its assigned cluster by comparing its distance to points in other clusters?

    <p>Silhouette method (A)</p> Signup and view all the answers

    Which of the following describes what occurs at the elbow point during the Elbow method analysis?

    <p>The reduction in SSE becomes less significant (B)</p> Signup and view all the answers

    Which of these factors is NOT considered when calculating the silhouette coefficient?

    <p>Distance to the farthest point in the cluster (A)</p> Signup and view all the answers

    What does hierarchical clustering primarily create for categorizing data?

    <p>A dendrogram (D)</p> Signup and view all the answers

    In hierarchical clustering, which policy involves starting with individual samples and merging them into groups?

    <p>Bottom-up policy (D)</p> Signup and view all the answers

    What does the root in a hierarchical clustering dendrogram represent?

    <p>The only cluster of all samples (D)</p> Signup and view all the answers

    What is the result of 'cutting' the dendrogram at a specified depth?

    <p>Creation of k groups of smaller dendrograms (B)</p> Signup and view all the answers

    Which type of hierarchical clustering divides clusters into smaller groups rather than merging them?

    <p>Divisive clustering (A)</p> Signup and view all the answers

    What kind of clustering structure is most commonly used in hierarchical clustering?

    <p>Tree-like structure (A)</p> Signup and view all the answers

    Which of the following statements accurately describes the leaves of a dendrogram in hierarchical clustering?

    <p>They represent clusters of single samples (B)</p> Signup and view all the answers

    Which of the following best defines a dendrogram?

    <p>A diagram that shows the structure of hierarchical clustering (B)</p> Signup and view all the answers

    Flashcards

    k-means clustering

    An algorithm that groups data points into clusters based on minimizing distances to cluster centers.

    Clusters

    Groups of data points with high similarity within the group.

    Centroids

    Data points representing the center of a cluster.

    Expectation-Maximization

    Two-step process of assigning data points to clusters then recalculating cluster centers.

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    within-cluster variances

    How spread out the data points are inside a cluster.

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    SSE

    Sum of Squared Errors, measure of error in k-means clustering.

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    k

    The number of clusters to create.

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    Euclidean distances

    Straight-line distances between data points.

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    Elbow Method

    A technique used in k-means clustering to determine the optimal number of clusters by finding the 'elbow point' on a graph of SSE (Sum of Squared Errors) against the number of clusters.

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    SSE (Sum of Squared Errors)

    A measure of the total error in k-means clustering. It calculates the sum of squared distances between each data point and its assigned cluster centroid.

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    Silhouette Coefficient

    A metric used to evaluate cluster quality by measuring how well each data point fits into its assigned cluster compared to other clusters.

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    Cluster Cohesion

    The degree to which data points within a cluster are similar or tightly grouped together.

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    Cluster Separation

    The degree to which different clusters are distinct or well-separated from each other.

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    Optimal Number of Clusters

    The ideal number of clusters that balances the need for minimal error (good fit) and a reasonable number of clusters for interpretability.

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    Trade-off Between Error and Clusters

    The balance between minimizing errors in k-means clustering and keeping the number of clusters manageable for understanding the data.

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    K = 3 (Elbow Point)

    The number of clusters determined by looking for the 'elbow point' in the graph, where the SSE curve begins to flatten.

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    Hierarchical Clustering

    A method that groups data points into a tree-like structure based on their similarity. It can use either a 'bottom-up' or 'top-down' approach to form clusters.

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    Agglomerative Clustering

    A 'bottom-up' approach to hierarchical clustering where data points are initially separated and gradually merged into larger clusters based on their similarity.

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    Divisive Clustering

    A 'top-down' approach to hierarchical clustering where the entire dataset is initially a single cluster and is progressively split into smaller clusters based on dissimilarity.

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    Dendrogram

    A tree-like diagram representing hierarchical clustering structure. It shows how clusters are formed and their relationships.

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    What is the purpose of a dendrogram?

    A dendrogram visually represents the hierarchical clustering results, showing the relationships and formation of clusters at different levels. It helps analyze the data's inherent structure and identify optimal groupings by cutting the tree at specified depth.

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    How are clusters assigned in a dendrogram?

    By cutting the dendrogram at a specific level, you get k groups of smaller dendrograms, which represent the final clusters. The cut level determines the number of clusters and their composition.

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    What makes a silhouette coefficient high?

    A high silhouette coefficient indicates that data points are well-clustered, meaning they are more similar to members of their own cluster than to members of other clusters.

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    How are cluster divisions determined in hierarchical clustering?

    Divisions are based on similarity (agglomerative) or dissimilarity (divisive) between data points. Algorithms like Ward's method or single linkage are used to measure these distances and determine the optimal groupings at each level of the hierarchy.

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    Study Notes

    CS 312 Introduction to Artificial Intelligence: Clustering Algorithms

    • Machine Learning Algorithm Overview: Machine learning algorithms are categorized into supervised learning (classification, regression), unsupervised learning (clustering), and other methods.
    • Clustering Algorithms: These algorithms group similar data points together. Unsupervised learning algorithms are used to automatically classify unlabeled data.
    • k-means Clustering: This algorithm takes the number of clusters (k) and a dataset as input, producing k clusters with minimized within-cluster variances. High similarity within clusters and low similarity between clusters are key characteristics. This algorithm uses expectation-maximization (two-step): expectation step assigns points to nearest centroid; maximization step computes new centroids.
    • k-means Algorithm Steps:
      • Specify the number of clusters (k).
      • Randomly initialize k centroids.
      • Repeat until centroids don't change:
        • Assign each point to its closest centroid.
        • Compute new centroids (mean of each cluster).
    • Choosing the Appropriate Number of Clusters (k):
      • Elbow Method: Plots SSE (Sum of Squared Errors) against k. The 'elbow' point suggests a good trade-off between error and the number of clusters.
      • Silhouette Coefficient: A value between -1 and 1. Higher values represent better-defined clusters. Higher values indicate samples are closer to their own clusters than to others.
    • Hierarchical Clustering: Creates a tree-like structure called a dendrogram, where clusters are formed at different levels. There are two types of Hierarchical clustering:
      • Agglomerative: Bottom-up approach, where similar data points are merged into clusters.
      • Divisive: Top-down approach, where a large cluster is split into smaller clusters at each stage.
    • Density-Based Clustering: Identifies clusters based on the density of data points in a region. This approach finds clusters of arbitrary shapes, unlike k-means which typically finds spherical clusters.
    • Reporting for Next Meeting:
      • Assigned Reporter 1: Provide sample code for k-means clustering, showing the method used to choose the number of clusters (k).
      • Assigned Reporters 3: Discuss Density-based clustering, compare it to k-means and hierarchical clustering, and present sample code for the three clustering algorithms with a common dataset, comparing and interpreting the results of each approach.

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    Description

    This quiz explores clustering algorithms within the context of CS 312 Introduction to Artificial Intelligence. It covers the basics of machine learning, specifically focusing on unsupervised learning techniques like k-means clustering. You'll learn about the steps involved in the k-means algorithm and its key characteristics.

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