We need to address eh variation so that it falls in the required pattern. The variation is due to mainly of 2 types. Common Cause variation and Special Cause Variation.
If the variation in output is caused only by common causes, the output will vary in a normal and predictable manner. In such cases, the process is said to be “stable” or “in a state of Statistical Control”. While the individual measurements may differ from each other, they tend to follow a Normal Distribution.
The normal distribution is characterized by the following
- Location (Typical Value)
- Spread – Amount by which the smaller values differ from the center.
The shape of the distribution will deviate from the normal curve in case of any un usual occourances. These changes can be called as Assignable causes.
The above findings will lead us to the sixth principle of SPC – Variation due to assignable causes tend to distort the normal distribution curve.