Some pronunciation studies focus specifically on the instruction of supra-segmental. Derwing, Munro and Wiebe (1997) conducted research in which ESL learners who had been studying for an average of ten years, participated in a speaking improvement course that focused on the supra-segmental features of pronunciation (e.g. stress, rhythm, intonation). Thirty-seven native listeners transcribed speech samples (true/false sentences) taken at the beginning of a 12-week course in order to assess the learners’ intelligibility. Each sample was rated in order of comprehensibility and degree of accentedness. In the end, there was a significant improvement in the intelligibility, and better ratings over time of comprehensibility and accentedness. They showed that 30 language learners could alter their pronunciation in a reading task (Derwing & Rossiter, 2003).