Abstract
Nozari and Whitt proposed an indirect approach to estimating the average production interval (i.e., the length of time between starting and finishing work on each good or defect-free product), based on measurements of the work-in-process inventory and the production rate. A necessary and sufficient condition for the consistency or validity of their estimator is derived herein. Our validity condition also yields an alternate estimator for the average production interval; as a direct estimator, it has better asymptotic efficiency than the indirect estimator.
