Controlling variation is an important aspect of quality improvement. Deming distinguishes between common cause variation and special cause variation and argues that both types of variation frequently result from people participating in the process. Confusing common cause and special cause variation can lead to incorrect decisions. This article analyzes the impact of an individual's ability to distinguish between common cause and special cause variation by simulating a manufacturing system with several human operators and a production manager. We use a recognition primed decision (RPD) model to simulate how human operators and the production manager would interpret the variation and make decisions to reduce the variation. A shared mental model with the RPD framework describes the interactions between different operators and the production manager. Results from this simulation demonstrate the importance of distinguishing between common cause and special cause variation, especially when problems occur at bottleneck points in the manufacturing system.