The error rate for a task is the frequency or number of errors that a user encounters whilst performing the task. This is one of the metrics used to evaluate the usability of a system.
Also used in hcistats2e: Chap. 1: pages 5, 6; Chap. 4: pages 45, 49; Chap. 5: pages 61, 62; Chap. 6: pages 66, 68; Chap. 7: page 86; Chap. 14: pages 169, 178, 179