Variation : Common and Special Causes

No two products or characteristics are exactly alike,because any process contains many sources of variability.The differences among products may be larger,or they may be immeasurably small,but they are always present.The diameter of a machined shaft,for instance,would be susceptible to potential variation from the machine (clearance, bearing wear),tool (strength, rate of wear),material(diameter,hardness),operator(part feed,accuracy of centering),maintenance (lubrication, replacement of worn parts),and environment (temperature,constancy of power supply).

Some sources of variation in the process cause short-term,piece-to-piece differences eg.,backlash and clearance within a machine and its fixturing, or the accuracy of a bookkeeper's work.
Other sources of variation tend to cause changes in the output only over a longer period of time,either gradually as with tool or machine wear,step-wise as with procedural changes,or irregularly,as with environmental changes such as power surges.Therefore,the time period and conditions over which measurements are made will affect the amount of the total variation that will be present.

The distribution can be characterized by

1-Location
2- Spread (span of values from smallest to largest)
3-Shape (the pattern of variation-whether it is symmetrical,skewed,etc.)

Common causes refer to the many sources of variation within a process that has a stable and repeatable distribution over time.This is called "in a state of statistical control".Common causes behave like a stable system of chance causes.If only common causes of variation are present and do not change,the output of a process is predictable.

Special causes (often called assignable causes) refer to any factors causing variation that are not always acting on the process.That is,when they occur,they make the (overall) process distribution change.Unless all the special causes of variation are identified and acted upon,they will continue to affect the process output in unpredictable ways.If special causes of variation are present,the process output is not stable over time.