The X-bar R chart is a type of control chart that helps the team to visualize and monitor (and sometimes control) the behavior of the variation in a process.
Use: There are two ways to make a bad part or make for an unhappy customer. First, if the centering of the variation in a process gets too close to either the upper specification limit or the lower specification limit, a bad part will be produced, or the customer will be unhappy. Secondly, if the variation in the process is centered, but the variation in the process is too wide, no matter how the team tries to center the variation, a bad part will be produced. The process variation overlaps either the upper specification limit or the lower specification limit. The X-bar R Chart monitors the process across time and reveals assignable-cause variation, namely looking out for these two conditions as they arise. The other value of using this chart is that if there is no assignable-cause variation, the process is to be left alone. If adjustments are made to the centering on a process that is exhibiting common-cause variation, more variation will be generated than it would had the process just been left alone. In other words, if the process is in statistical control, leave it alone.
