Quality Metrics Questions

#1
Hi

would anyone cares to share there views and opinions on the below two questions related to Quality Metrics. Any feedback would be appreciated.

Q1). What are the Pros and Cons of measuring each, Defects per million opportunities (Dpmo), Sigma Quality Leve (SQL) and Rolled Throughput Yield (RTY ) ?
Q2). which of the above 3 should I be looking at measuring if want a true and accurate measure of how good or bad the quality is for our manufacturing ?

Thank you in advance
 
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optomist1

A Sea of Statistics
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#2
details, back ground (field, product, process...) and context will help get this answered (better)
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#3
Optimist is correct. More background will provide us the context to give you a more specifically helpful answer. But in general:

DPMO is fraught with gamesmanship as people argue about the number of opportunities. The concept is good but actually doing it is incredibly difficult unless you have an incredibly simple product and manufacturing process. Unless of course you mean the very simple ppm which is just teh number of defects divided by the number of parts times some ‘scaling’ number. (X100 = percent, X1M = ppm...)

Sigma quality level is just stupid as it is a transformation of actual results based on teh theoretical - and mythical - Normal distribution. And if you intend to include the thoroughly debunked 1.5sigma shift you might as well just randomly makeup numbers. And are you going to be using categorical data (defects) or continuous data (actual measured values vs. tolerances)? Or a mix of both? Sigma levels have NO place with categorical data.

RTY has value. IF you properly apply it. It is a decent high level metric. I’ve used it successfully for years in assembly operations. The critical part is how to get people to acknowledge and understand that it involves first time ‘pass’ and not just scrap or rework. When properly applied it exposes the hidden rework factory that too many believe is just what you need to do to get stuff to pass. But that si the role of the quality professional.
 
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