Summary - P.807 (2016) Cor. 1 (07/2026) - Correction to scoring formula in clause 6.4

Recommendation ITU-T P.807 describes a subjective testing methodology for assessing speech intelligibility in communications settings, systems and devices. The method provides a percent correct intelligibility score based on a two-alternative, forced-choice task where the stimulus is one of the two words from a pair of words, i.e., a test item. Half of the test items are rhyming word-pairs (i.e., they differ only in the initial consonant) and half are alliterative word-pairs (i.e., they differ only in the final consonant). The two critical consonants in each test item differ only in a single distinctive feature (see Annex A for a description of distinctive features). In addition to a score for overall intelligibility, the method provides scores for each of six distinctive features: voicing, nasality, sustention, sibilation, graveness and compactness. These scores may be used to diagnose the specific cause of impairments leading to degradation of speech intelligibility.
Corrigendum 1 corrects an imprecision in the scoring formula presented in clause 6.4, for computing speech intelligibility scores adjusted for guessing. The original formula works correctly for two-alternative forced-choice tests (n=2) only. When applied to tests with more than two response alternatives, such as the Modified Rhyme Test (MRT; n=6), the formula significantly overcorrects for chance performance.