Which one of the following categories best explains why the number of incoming calls at a service center varies slightly day-to-day?

The variation in the number of incoming calls at a service center from day to day is best categorized as expected or common cause variation. This type of variation is inherent in any process and results from a multitude of small, everyday influences that are natural to the system. Factors such as fluctuations in customer behavior, seasonal trends, or even day-of-the-week effects contribute to this common cause variation, which typically falls within a predictable range.

Common cause variation differs from special cause variation, which would occur due to specific, identifiable changes to the process or environment—for example, a new marketing campaign or temporary staffing shortages. Therefore, while variations can occur because of special circumstances, the subtle day-to-day changes in call volume are most accurately explained by inherent variability in the service center's operations, aligning with the characteristics of common cause variation.