Understanding Variation-Part 2: The Control Chart

Both run and control charts can be used to answer the question "How will we know that a change is an improvement?". Run charts are advantageous at the start of an improvement effort, when little is known about the system and how it performs. Over time, transitioning to a control chart may provide more specific information and insight into the data being measured. The type of numeric data being collected helps guide the most appropriate choice of control chart. Control chart rules exist and should be used to determine if a change in the data represents common cause or special cause variation. Similar to run charts, it is important not to over- or under-react to a single point on a control chart. Control chart rules help quality improvers determine whether interventions being tested are having a meaningful impact on the system.