We Need to Talk About Classification Evaluation Metrics in NLP
Peter Vickers Loïc Barrault Emilio Monti Nikolaos Aletras
In Natural Language Processing (NLP) classification tasks such as topic
categorisation and sentiment analysis, model generalizability is generally
measured with standard metrics such as Accuracy, F-Measure, or AUC-ROC. The
diversity of metrics, and the arbitrariness of their application suggest that
there is no agreement within NLP on a single best metric to use. This lack
suggests there has not been sufficient examination of the underlying heuristics
which each metric encodes. To address this we compare several standard
classification metrics with more 'exotic' metrics and demonstrate that a
random-guess normalised Informedness metric is a parsimonious baseline for task
performance. To show how important the choice of metric is, we perform
extensive experiments on a wide range of NLP tasks including a synthetic
scenario, natural language understanding, question answering and machine
translation. Across these tasks we use a superset of metrics to rank models and
find that Informedness best captures the ideal model characteristics. Finally,
we release a Python implementation of Informedness following the SciKitLearn
classifier format.