Remarks on queue inference from departure data alone and the importance of the queue inference engine

Recently a generalization of Larson’s Queue Inference Engine (Q.I.E.) for computing conditional expected queue length was proposed in this journal. Although the conditional expectation calculation is correct and of independent interest, it does not represent the queuing inference quantities claimed. We demonstrate this with two examples with identical mean service times where the expectations claimed differ. If customer departure times are included, the examples demonstrate that queue inference cannot be made from departure data alone with unknown service distribution.