Sixth principle of SPC – causes for Variation

According to the sixth principle of SPC  a frequency distribution will be deviating from normal distribution, only in the presence of any assignable cause.
A frequency distribution is a tally of measurements that shows the number of times the measurement is included int he tally. From this frequency distribution we can see if there are only chance causes present in the process of any assignable causes are acting.
If there is a distortion from the normal curve, we can say that there is presence of assignable causes. This finding can actually help us to find the causes and address them.
Various effects of the presence of assignable causes, will tend to distort the shape in center, or the spread as sees earlier. This indication forms the basis of various techniques used in Statistical Process Control.

Originally posted 2012-05-02 01:31:00.

Basic Principles of Statistical Process Control

The six principles below are the Basic Principles of Statistical Process Control (SPC). These can be clearly understood using Frequency Distributions.
The principles are listed below. The explanation is linked to each sentence

1. No two things are exactly alike
2. Variation in a product or process can be measured
3. Things Vary according to a definite pattern
4. Whenever things of the same kind are measured, a large group of the measurements will tend to cluster around the middle.
5. It is possible to determine the shape of the distribution curve for the measured Output (Parts produced, transaction) by any process
6. Variation is due to assignable causes tend to distort the Normal distribution Curve.
To Understand more on these principles, we can study the data from the output of the process. These foundation principles will be useful for all types of processes.