Common Cause and Special Cause Variation

Suzuki San continues " Dear Friends , This time I am going to delve into a bit of technical aspects of Quality .

We will look into some aspects that affect Quality . When I ask what is Quality , many of you will say that Quality is conforming to specifications or addressing customers requirements , etc....

You will notice that when you manufacture a product or provide a service , a particular item or service may satisfy one customer but not necessarily the other even if its the same product or service . This happens when the product or service delivery varies in its specifications .

So we have learnt earlier that Variation is one of the main enemies of Quality and that if a specification is defined , one must ensure that the specification is met consistently . This factor in manufacturing statistical terms is called as REPEATABILITY . The ability of the process to produce the same result again and again .

When there is variation , the results keep varying and hence will produce poor quality , Yes ?? But in a process some degree of variation will happen as one cannot have a perfect output every time .

Variation is caused due to different factors and fall under two major categories .

One is COMMON CAUSE VARIATION and the other is SPECIAL CAUSE VARIATION .

SPECIAL CAUSE VARIATION :

This variation is an unusual happening arising out of an unusual occurrence . Some examples to make one understand is

1. In a production line , if the tool tip breaks and it damages the component under production , those components will not conform to the desired output and will be rejected . This defective is due to a Special cause .... the breakage of the tool tip .

2. When you are going to office by a bus everyday to reach at 07:30hrs  , and on one particular day an accident happens on way and you are delayed  due to this , your delay is due to a special cause .

3. When you have a new staff taking charge of Telephones and he / she makes numerous mistakes , these are caused due to special causes .

COMMON CAUSE VARIATION:

This variation is very natural and will occur in any process irrespective of its speciality or simplicity . This variation is fully expected and is common and natural .

Examples of Common cause variation is

1. Try putting your signature 10 times on a sheet of paper . You will notice that these signatures are fully yours but will contain certain minor variations between each . This is due to common cause variations .

2. Taking the example above of coming to office every day when you say leave your home by 06:30 hrs sharp , and you reach office between say 07:35 to 07:45 hrs , it can be attributed to common cause variations .

3. When milk bottling company is bottling milk in 2 litre containers , an accurate measure of these cans may reveal that the bottles are filled between 1.97 litres to say 2.07 litres and never perfect to 2 litres . This can be attributed to common cause variations .

Most of the common causes happened due to factors of variation caused by Man , Machine , Method , Material , Mother Nature ( temperature etc..) , Measurement etc...





In the above graph , one will find that if 98% was the desired output , due to common cause variations the readings everyday hover around 98% . However due to a special cause on 23/1 , the output had decreased to 81% . This shows the difference between common and special cause variations . 

Now , I hope you have understood something on what are the causes of variation . What I want you to tell me is that , out of Special Causes and Common cause , which one is very important and why ?

Please send your responses to my good friend KJ on his email id kj@specialised.com.zm

I will explain more in the coming days . Thanks "

In a jiffy , Suzuki San was gone .

Note 1 : Suzuki San , Miss P and her company are characters  developed by KJ and has no relevance to any particular person / processes/situation    . All mention are purely for illustration purpose of the central theme of the case and are personal opinions unless cited and shall have no recourse of consequences to author . Readers can use their practical inputs to alter any of the material that may be published . Readers are invited to express their views on this continuing case study series .

Note 2: Some of the readers have mentioned that they are unable to place their comments . Please click on the "No comments"  link , and make your comments as an ANONYMOUS user if you are not logged into your google account  , though it will be nice if we can have your name details in the comments .If you are still not able to send your comment on the post , please email kj@specialised.com.zm