Before we do anything about the process, we must understand it and know the characteristics of what we are about to do. We must understand the difference between variable and attribute data because the definition of the data will guide us in the appropriate sampling as well as the appropriate selection of the control chart form.
There are two kinds of data: variable (measurable) and attribute (go-no go). With variable data, one has maximum opportunity and flexibility to study the process with a variety of control charts . On the other hand, with attribute data, both the opportunity and flexibility are much less. In addition, if you deal with attribute data, the power of the data is increased with many samples, whereas with variable data, that power is sufficient with far less sampling.