Before moving on to the next section, we should note that the charts mentioned so far are for use with variable data. Characteristics which can be measured provide variable data. Length, diameter, breaking strain, etc. are all examples of variables. In certain cases, it is not possible to measure a particular quality characteristic. Such characteristics are called attributes, and they can only be counted. Examples of attributes include the number of discolourations in a length of cloth, or the number of defective items in a sample.

For more information about the difference between variable and attribute data, or information about the different chart types, you might like to look at the larger SPC course. That course has information about how to plot, and how to prepare each of the major charts used in SPC.