Evaluating the Performance of the Erlang Models for Call Centers
Thomas R. Robbins

Abstract
In this paper we evaluate two queuing models used to analyze call centers; the Erlang C and Erlang A models. The Erlang C is a simple model that ignores caller abandonment and is the most commonly used model. The Erlang A model allows for abandonment, but performance measures are more difficult to calculate. We compare the theoretical performance predictions of these models to a steady state simulation model where many of the simplifying assumptions used in standard analytical models are relaxed. Our findings support the assertion that the Erlang A model is more accurate, but we find that in contrast to the Erlang C model, Erlang A tends to be optimistically biased. Our findings indicate that neither model clearly dominates the other in all situations and that care must be taken to select the correct model based on call center conditions and the intended purpose of the model.

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