An Empirical Model of Acknowledgment for Spoken-Language Systems
  We refine and extend prior views of the description, purposes, and contexts-of-use of acknowledgment acts through empirical examination of the use of acknowledgments in task-based conversation. We distinguish three broad classes of acknowledgments (other-->ackn, self-->other-->ackn, and self+ackn) and present a catalogue of 13 patterns within these classes that account for the specific uses of acknowledgment in the corpus. 
David G. Novick (Oregon Graduate Institute), Stephen Sutton (Oregon   Graduate Institute)