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Common vs special causes of variation.

Posted: Tue Sep 12, 2017 4:54 am
by nirajsys@gmail.com
Hi,

What's the fundamental diff between common and special cause of variations?

Also,how to decide on which tools to use for identifying common and special causes respectively?

Pls advise.

Re: Common vs special causes of variation.

Posted: Tue Sep 12, 2017 12:51 pm
by nirajsys@gmail.com
Can group members advise on the above post?

Re: Common vs special causes of variation.

Posted: Wed Sep 13, 2017 8:40 pm
by manishpn
following threads may help you
viewtopic.php?f=3&t=3922&p=15306&hilit=special+cause#p15306
viewtopic.php?f=3&t=6810&p=24808&hilit=special+cause#p24808

One should remember that common cause is “common” and that is not considered a defect while special cause is caused by a “special” reason (defect) which must be resolved in order for the process to run probably.

e.g.

Dairy get regular supply of milk from various farmers. normal range of fat % from milk (knowing the breed of cow in the region) is between 3-5%
So when dairy receives milk supply they will measure the fat% for each batch and as long as fat is within range its ok it considered as common cause, say sometimes fat may be 3.6%, 4%, 4.3% etc and no action needs to be taken

however if the batch has 2% fat then its a special cause cause then some action has to be taken

Re: Common vs special causes of variation.

Posted: Wed Sep 13, 2017 11:32 pm
by nazrulq
If seven readings for example 3.6%,3.8%,3.9%,3.1%,3.2%,3.5%,3.7% below the mean. Then there is a problem in dairy fat analysis system.Should be investigated!

Re: Common vs special causes of variation.

Posted: Thu Sep 14, 2017 3:23 am
by nirajsys@gmail.com
Thanks Manish & Nazrul