Many people complain about variables they can not control saying 'there is nothing we do!' With a Taguchi Robust Design of Experiment technique there is a lot you can do to make your process Robust to that variation...This is a great approach that you should be covering on your 6 Sigma training...

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BOOK LINKS
Drink tea and read the paper - https://www.lulu.com/en/us/shop/paul-...

Design of Experiments for 21st Century Engineers - https://www.lulu.com/en/us/shop/paul-...

Statistical Process Control for Small Batch Production - https://www.lulu.com/en/gb/shop/paul-...