Rico Richter

Daniel Atzberger, Tim Cech, Willy Scheibel et al.

Topic models are a class of unsupervised learning algorithms for detecting the semantic structure within a text corpus. Together with a subsequent dimensionality reduction algorithm, topic models can be used for deriving spatializations for text corpora as two-dimensional scatter plots, reflecting semantic similarity between the documents and supporting corpus analysis. Although the choice of the topic model, the dimensionality reduction, and their underlying hyperparameters significantly impact the resulting layout, it is unknown which particular combinations result in high-quality layouts with respect to accuracy and perception metrics. To investigate the effectiveness of topic models and dimensionality reduction methods for the spatialization of corpora as two-dimensional scatter plots (or basis for landscape-type visualizations), we present a large-scale, benchmark-based computational evaluation. Our evaluation consists of (1) a set of corpora, (2) a set of layout algorithms that are combinations of topic models and dimensionality reductions, and (3) quality metrics for quantifying the resulting layout. The corpora are given as document-term matrices, and each document is assigned to a thematic class. The chosen metrics quantify the preservation of local and global properties and the perceptual effectiveness of the two-dimensional scatter plots. By evaluating the benchmark on a computing cluster, we derived a multivariate dataset with over 45 000 individual layouts and corresponding quality metrics. Based on the results, we propose guidelines for the effective design of text spatializations that are based on topic models and dimensionality reductions. As a main result, we show that interpretable topic models are beneficial for capturing the structure of text corpora. We furthermore recommend the use of t-SNE as a subsequent dimensionality reduction.

Tim Cech, Ole Wegen, Daniel Atzberger et al.

Standard datasets are frequently used to train and evaluate Machine Learning models. However, the assumed standardness of these datasets leads to a lack of in-depth discussion on how their labels match the derived categories for the respective use case, which we demonstrate by reviewing recent literature that employs standard datasets. We find that the standardness of the datasets seems to cloud their actual coherency and applicability, thus impeding the trust in Machine Learning models trained on these datasets. Therefore, we argue against the uncritical use of standard datasets and advocate for their critical examination instead. For this, we suggest to use Grounded Theory in combination with Hypotheses Testing through Visualization as methods to evaluate the match between use case, derived categories, and labels. We exemplify this approach by applying it to the 20 Newsgroups dataset and the MNIST dataset, both considered standard datasets in their respective domain. The results show that the labels of the 20 Newsgroups dataset are imprecise, which implies that neither a Machine Learning model can learn a meaningful abstraction of derived categories nor one can draw conclusions from achieving high accuracy on this dataset. For the MNIST dataset, we demonstrate that the labels can be confirmed to be defined well. We conclude that also for datasets that are considered to be standard, quality and suitability have to be assessed in order to learn meaningful abstractions and, thus, improve trust in Machine Learning models.