The variable that is chosen for Mean and Run charts must be measurable and expressed in numbers. Such kinds of variable are called quality characteristics. Quality characteristics can be expressed in seven basic units (Length, Mass, Time, Electric current, Temperature,or Luminous Intensity) as well as derived units such as Power, Velocity, Force, Energy, Density, and Pressure etc.)
First attention would normally be given to those quality characteristics which affecting the performance of the product. Cost saving opportunities involves situation where spoilage and rework costs are high. Here Pareto analysis is useful for establishing priorities.
In any organization, a large number of variables make up product or service. It is therefore impossible to place Mean and Range charts on all variables. A judicious selection of those quality characteristic is required.