Average linkage

Definition

Average linkage is a method used in hierarchical clustering that calculates the distance between clusters based on the average distance between all pairs of objects in the clusters. This approach helps to form clusters by merging them based on the overall similarity of their constituent objects, rather than relying on extreme values or single points. Average linkage is a compromise between single linkage, which uses the minimum distance, and complete linkage, which uses the maximum distance.

5 Must Know Facts For Your Next Test

1. Average linkage is often preferred because it balances sensitivity to outliers and provides a more robust measure of cluster similarity compared to single or complete linkage.
2. In average linkage, if two clusters have many close points, they will be merged even if their furthest points are far apart.
3. The average distance is calculated by taking the sum of all distances between pairs of points from different clusters and dividing by the total number of pairs.
4. This method can produce different cluster shapes than other methods due to its reliance on overall distances rather than extremes.
5. Average linkage tends to create more compact clusters, which can improve the interpretability of results in various applications.

Review Questions

• How does average linkage differ from single and complete linkage methods in hierarchical clustering?
  • Average linkage calculates the distance between clusters based on the average distance between all pairs of objects, making it less sensitive to outliers compared to single linkage, which focuses on minimum distances. In contrast, complete linkage uses maximum distances, often leading to elongated clusters. This difference affects how clusters are formed and can result in varying shapes and sizes in the final clustering output.
• Discuss how average linkage can influence the shape and compactness of clusters formed during hierarchical clustering.
  • Average linkage influences cluster shape by considering all pairwise distances when calculating similarity, leading to more compact and spherical clusters. This averaging can mitigate issues with outliers, which might disproportionately affect results in single or complete linkage approaches. As a result, average linkage typically yields clusters that are better structured and more interpretable, allowing for clearer insights into data organization.
• Evaluate the advantages and disadvantages of using average linkage in hierarchical clustering compared to other methods.
  • Using average linkage has several advantages, such as producing compact clusters and reducing sensitivity to outliers. However, one disadvantage is that it may lead to merging clusters that are not intuitively similar due to its reliance on averages. In contrast, single and complete linkage methods can create more distinct separations between clusters but may be overly influenced by extreme values. Therefore, choosing average linkage can enhance interpretability while balancing cluster cohesion and separation.

Related terms

Hierarchical Clustering: A clustering method that builds a hierarchy of clusters either through a divisive approach or an agglomerative approach.

Single Linkage: A clustering method that defines the distance between two clusters as the minimum distance between any single pair of points from each cluster.

Complete Linkage: A clustering method that defines the distance between two clusters as the maximum distance between any single pair of points from each cluster.