The Voice of the Process

Control charts enable us to see what is happening, clearly and immediately. What they show us is what the process is actually doing, not what we thought it was doing, nor what we would like it to do. For this reason, control charts are often referred to as the voice of the process.

This term is often used to highlight the distinction between control limits (voice of the process), and specification limits, which are referred to as the voice of the customer. This distinction is fundamental to SPC. Customer specification limits will tell you no more than what the customer's minimum requirements are from the process. Only the process itself, speaking through control charts can tell you whether it is meeting, failing, or (ideally) greatly exceeding these requirements.

Control charts are completed by:

  1. Taking random samples from the process at regular intervals and recording information on the characteristic under investigation
  2. Performing some straightforward calculations on the data (e.g. calculating an average)
  3. Plotting the calculated figures on a graph
  4. Linking consecutive plots together