Operations Management - Waiting Line Theory

What is a Waiting Line

Managers use waiting line theory to help with capacity and efficiency. A waiting line is one or more customer or items queued for an operation, which can include people waiting for service, materials waiting for further processing, equipment waiting for maintenance, and sales orders waiting for delivery.

Waiting lines help to reduce capacity related- costs by accommodating short rushes of demand; Managers are able to control these short surges with less capacity. As waiting lines increase, capacity decreases. However, as these waiting lines increase, customer related cost increase as well, as their expectations are not met. Therefore, managers must find the perfect balance when it comes to the type of waiting line they use to decrease monetary costs as well as customer related costs.

Structure of Waiting Lines

Momentary imbalances between capacity and demand create the need for waiting lines. Four elements apply to all situations:

1. Customer population that generates potential demand

2. A waiting line of customer or items

3. A workstation or operation, consisting a person, a machine, or both necessary to perform one or more activities

4. A priority rule, which selects the next customer to be served or item to be transformed by a workstation or operation

Configuration

Waiting lines have different arrangements to accomplish different things. Lines are either single or multiple. Single lines usually are used in situations where multiple workstations are available, each handling general transactions. The single line set up keeps the workstations busy and distributes services fairly among customers. This is most effective when all operations demanded by a customer can be performed by a single workstation. The multiple line arrangement works best in situations where workstations provide a limited set of services. This way, customers can select the line that is geared specifically for their needs.

Priority Rule

The priority rule determines which customer or item to serve next. The majority of configurations use the first-come, first-served rule, where priority is given to those that enter the line up first. However, many configuration utilize a pre-emptive discipline, which is a rule allowing a person of higher priority to receive service ahead of other customers, regardless of who entered the line up first. For example, some airports provide a wheelchair line at security checkpoint where individuals with disabilities are able to bypass the long line ups.

Using Waiting Line Models to Analyze Operations

Waiting line models can be useful in determining the trade offs in costs of waiting and potential gains from reducing capacity. There are five characteristics of performance that managers should be concerned with: Line length, number of customers in the line, waiting time of line, total time in the system, and capacity utilization. When looking at line length, the number of customers waiting in line can indicate either one of two situations. Short line ups can translate as good customer service, or it could mean too much capacity. Alternatively, long line ups can indicate poor service, or not enough capacity utilization. The number of customer in a line up also relate to the process efficiency and capacity. Waiting time in the queue can affect a customer’s perceived quality of the experience. Typically customer are more satisfied and with quick service and short waiting times. Total time in the system (the moment a customer enters until the moment they exit) can also reflect efficiency and capacity. If customers are spending too much time in the system, managers need to analyze where the loss of time is occurring, and make necessary changes to reduce time. Lastly, with capacity utilization, management’s goal is to have high utilization and profitability while maintaining good operating characteristics.

References

Ritzman, L. (2007). Waiting Lines. G. Bennett. Foundations of Operations Management (pp. 123-129). Upper Saddle River, NJ: Pearson Education