Poor estimates of context are worse than none

It is difficult to estimate the probability of a word's context because of sparse data problems. If appropriate care is taken, we find that it is possible to make useful estimates of contextual probabilities that improve performance in a spelling correction application. In contrast, less careful estimates are found to be useless. Specifically, we will show that the Good-Turing method makes the use of contextual information practical for a spelling corrector, while attempts to use the maximum likelihood estimator (MLE) or expected likelihood estimator (ELE) fail. Spelling correction was selected as an application domain because it is analogous to many important recognition applications based on a noisy channel model (such as speech recognition), though somewhat simpler and therefore possibly more amenable to detailed statistical analysis.