Error Classification and Analysis for Machine Translation Quality Assessment
This chapter presents an overview of different approaches and tasks related to classification and analysis of errors in machine translation (MT) output. Manual error classification is a resource and time intensive task which suffers from low inter evaluator agreement, especially if a large number of error classes have to be distinguished. Automatic error analysis can overcome these deficiencies, but state of the art tools are still not able to distinguish detailed error classes, and are prone to confusion between mistranslations, omissions, and additions. Despite these disadvantages, automatic tools can efficiently replace human evaluators both for estimating the distribution of error classes in a given translation output, as well as for comparing different translation outputs. They can also facilitate manual error classification by pre annotation, since correcting or expanding existing error tags requires less time and effort than assigning error tags from scratch. Classification of post editing operations is more convenient both for manual and for automatic processing, and also enables more reliable assessment of automatic tools. Apart from assigning error tags to incorrectly translated (groups of) words, error analysis can be performed by examining unmatched sequences of words, part of speech (POS) tags or other units, as well as by identifying language related and linguistically motivated issues. These linguistic categories can be then used to perform automatic evaluation specifically on these units, or to analyse their frequency and nature. Due to its complexity and variety, error analysis is an active field of research with many possible directions for development and innovation.