For example, in a much simplified error correction task concerning only the correction of prepositions, Tetreault and Chodorow (2008) showed an actual difference of 10% precision and 5% recall between two annotators.
the probability of a learner n-gram is compared with the probability of a candidate corrected ngram, and if the difference is greater than some threshold, an error was perceived to have been detected and a higher scoring replacement n-gram could be suggested.