Center line

A horizontal reference line on a control chart that is the average value of the charted quality characteristic. If a process is in control, the points will randomly vary around the center line.

Use the center line to see how the process performs compared to the average. For example, the center line in the above Xbar chart represents the average weight of samples of spaghetti sauce cans over time. The food company wants to ensure that the actual weight of the cans is close to the advertised weight on the can's label. Most samples are just above or below the centerline, indicating that the samples are close to the average weight.

Do not confuse the center line with the target value for your process. The target is your desired outcome, not the actual outcome.