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The difference between assignable variation and random variation is defined.

Assignable variation is the variation present in a process that arises due to a deliberate/external flaw or element.

For example, a deliberate change in the machine setting can result in defective products.

Here, the cause of variation is the alteration in the machine setting, and the resultant assignable variation is the defective product.

It is essential to alleviate the assignable cause of variation so that the process makes good and acceptable goods.

Random variation is the variation that occurs randomly in the process, mainly due to chance causes.

For example, periodic heating in an element of a machine results in a slight variation in the texture of the product.

Here, the cause of the random variation is the periodic heating of the machine, and the random variation is the slight change in the texture.

The random variations are inherently present in the process as no two goods/services produced can be perfectly identical. Thus, random causes of variation can be removed.

The main difference between the assignable variation and the random variation is that assignable occurs suddenly, is large and significant, and affects the process adversely. On the other hand,the random variation exists naturally, is not drastic, and does not significantly affect the process.