Statistical Process Control (SPC) is a powerful collection of  problem solving tools useful in achieving process stability and improving capability through the reduction of variability. Its seven major tools are:

  1. Histogram or stem-and-leaf plot
  2. Check sheet
  3. Pareto chart
  4. Cause-and-effect diagram
  5. Defect concentration diagram
  6. Scatter diagram
  7. Control chart.

Chance Causes of Variation

In any production process, regardless of how well designed or carefully maintained it is, a certain amount of inherent or natural variability will always exist. This natural variability or “background noise” is the cumulative effect of many small, essentially unavoidable causes. In the framework of statistical quality control, this natural variability is often called a “stable system of chance causes”. A process that is operating with only chance causes of variation present is said to be in statistical control. In other words, the chance causes are an inherent part of the process.

Assignable Causes of Variation

Other kinds of variability may occasionally be present in the output of a process. This variability in key quality characteristics usually arises from three sources : improperly adjusted or controlled machines, operator errors or defective raw material. Such variability is generally large when compared to the background noise, and it usually represents an unacceptable level of process performance. We refer to these sources of variability that are not part of the chance cause pattern as assignable causes of variation. A process that is operating in the presence of assignable causes is said to be an out-of-control process.