Inbound call center operations are challenging to manage, in large part because there is considerable uncertainty in estimates of arrival rates, which vary over time. We have developed a general purpose simulation model for inbound call center operations which supports time varying and uncertain arrival rates along with variable staffing. We outline the conceptual and technical design of the simulation model. We then define and conduct an initial experiment that uses the model to evaluate the impact of arrival rate uncertainty on call center performance. We find that arrival rate uncertainty creates significant planning challenges for managers attempting to satisfy tight performance targets, particularly one-sided performance measures. We also find that abandonment rate has a major impact on call center performance. Call centers are a large and growing component of the U.S. and world economy (Gans et al. 2003) . In 1999 an estimated 1.5 million workers were employed in call centers in the US alone. Large scale call centers are technically and managerially sophisticated operations and have been the subject of substantial academic research. A call center is a facility designed to support the delivery of some interactive service via telephone communications, typically an office space with multiple workstations manned by agents who place and receive calls (Gans et al. 2003) . Call center applications include tele-marketing, customer service, help desk support, and emergency dispatch. Most call centers support both inbound and outbound operations. An inbound operation takes customer initiated calls while an outbound operation places calls to customers. Inbound and outbound calls are often handled by separate teams of agents, though in some call centers inbound and outbound calls are mixed in a process known as blending. Even a moderately sophisticated call center is equipped with advanced computer and telecommunications equipment. An inbound call typically connects from the public service telephone network (PSTN) to the call center's switch, the private branch exchange (PBX), over a number of owned or leased trunk lines. Callers may initially connect to an Interactive Voice Response Unit (IVR) where the caller can use her keypad to select options and potentially provide data input to call center system. When callers need to speak to an agent, the call is handled by the Automated Call Distributor (ACD). The ACD routes calls internal to the call center and is responsible for monitoring agent status, collecting data, managing on hold queues, and making potentially complex routing decisions. For example, in call centers that employ skills based routing, a complex decision process is used to match callers and agents based on multiple criteria concerning both the callers and the agents. In addition to the telephone system, a call center agent usually has a computer terminal connected into one or more enterprise applications; these are typically classified under the general category of Customer Relationship Management (CRM). In lower volume, less sophisticated call centers the telecommunications systems and information processing systems are completely separate, but most moderately sophisticated centers use some form of Computer Telephony Integration (CTI). CTI allows information to pass back and forth between the two systems. The call center system typically has some form of CTI middleware to manage the integration of the telephony and computing systems. With that data the CTI system can orchestrate a screen pop, the simultaneous delivery of a call to an agent's telephone and a screen of information to the same agent's workstation. Gans et al. (2003) document a series of industry standard measures used to evaluate call center performance. We summarize the key measures here: • Excess Capacity: the minimum staff level required to provide queue stability is well known to be N = R i , where R i is the offer load; the ratio of EVALUATING ARRIVAL RATE UNCERTAINTY IN CALL CENTERS
e authors of REF are experimenting with the well-accepted arrival rate model for the call center to model and evaluate the impact of arrival rate uncertainty on the call center performance.
Evaluating arrival rate uncertainty in call centers
WSC '06
Computer Science