Stalking the Endangered CAT: A Queueing Analysis of Congestion at Automatic Teller Machines

A study to determine customer service standards for the automatic teller machines (ATM's) of a large retail bank led to the formulation and calibration of a finite waiting room M/M/c/K queueing model. The model suggested that the percentage of lost customers be adopted as the service standard instead of line wait. The new standard was applied against existing transaction reports to identify those congested ATM facilities that could best profit from additional machines and lobby space. The magnitude of the increased business at the upgraded facilities partly confirmed the model's hypothesis that substantial numbers of customers had indeed been balking.