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-allen/drink-tea-and-read-the-paper/paperback/product-1v474jqr.html
Design of Experiments for 21st Century Engineers - https://www.lulu.com/en/us/shop/paul-allen/design-of-experiments-for-21st-century-engineers/paperback/product-124en77d.html
Statistical Process Control for Small Batch Production - https://www.lulu.com/en/gb/shop/paul-allen/statistical-process-control-for-small-batch-production/paperback/product-7j9pde.html?page=1&pageSize=4