Common vs special causes of variation.

[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.

[nirajsys@gmail.com]

Can group members advise on the above post?

[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

[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!

[nirajsys@gmail.com]

Thanks Manish & Nazrul