Predictability effects on durations of content and function words in conversational English

In a regression study of conversational speech, we show that frequency, contextual predictability, and repetition have separate contributions to word duration, despite their substantial correlations. We also found that content- and function-word durations are affected differently by their frequency and predictability. Content words are shorter when more frequent, and shorter when repeated, while function words are not so affected. Function words have shorter pronunciations, after controlling for frequency and predictability. While both content and function words are strongly affected by predictability from the word following them, sensitivity to predictability from the preceding word is largely limited to very frequent function words. The results support the view that content and function words are accessed differently in production. We suggest a lexical-access-based model of our results, in which frequency or repetition leads to shorter or longer word durations by causing faster or slower lexical access, mediated by a general mechanism that coordinates the pace of higher-level planning and the execution of the articulatory plan.