Wasserstein t-SNE

TL;DR

This paper introduces Wasserstein t-SNE, a novel method for visualizing hierarchical datasets by embedding units based on Wasserstein distances that account for within-unit distribution shapes, demonstrated on synthetic and real election data.

Contribution

The paper develops a Wasserstein distance-based t-SNE approach that captures within-unit distribution shapes for hierarchical data visualization, including scalable exact and approximate computation methods.

Findings

Effectively uncovers meaningful structure in synthetic data.

Reveals insightful patterns in German election polling data.

Provides scalable methods for Wasserstein distance computation.

Abstract

Scientific datasets often have hierarchical structure: for example, in surveys, individual participants (samples) might be grouped at a higher level (units) such as their geographical region. In these settings, the interest is often in exploring the structure on the unit level rather than on the sample level. Units can be compared based on the distance between their means, however this ignores the within-unit distribution of samples. Here we develop an approach for exploratory analysis of hierarchical datasets using the Wasserstein distance metric that takes into account the shapes of within-unit distributions. We use t-SNE to construct 2D embeddings of the units, based on the matrix of pairwise Wasserstein distances between them. The distance matrix can be efficiently computed by approximating each unit with a Gaussian distribution, but we also provide a scalable method to compute exact Wasserstein distances. We use synthetic data to demonstrate the effectiveness of our Wasserstein t-SNE, and apply it to data from the 2017 German parliamentary election, considering polling stations as samples and voting districts as units. The resulting embedding uncovers meaningful structure in the data.