The coherence of a text is the degree to which the reader can describe the role of each individual sentence (or group of sentences) with respect to the text as a whole. Theories such as Rhetorical Structure Theory (Mann and Thompson, 1988) attempt to formalize coherence using a set of inter-segment relations (such as Cause, Solutionhood, Elaboration) that express the internal document structure.
Measurement of the total contextual coherence (T.C. Halliday in Van Slype's Critical Report).
Measure degree to which roles of each discourse unit can be identified with respect to a gold standard.
Vanni & Miller (2001, 2002), for example, measure this feature by counting the total number of sentences in the machine translated text to which RST labels can be assigned. See Mann & Thompson
It has been asserted that the quality of a translation can be assessed by its level of coherence without comparing it to the original text. Once a sufficiently large sample is available, the probability that the translation should be at the same time coherent and totally wrong is very weak. (Wilks in personal communication, 1992, also cited in Van Slype's Critical Report).
According to the definition the assessment of coherence can be done by a monolingual evaluator, whereas any judgement on the correctness of the translation necessarily involves making use of a bilingual evaluator. (Wilks in Van Slype's Critical Report).