# Quality toolbox decision tree

#### Tahirawan77

##### Starting to get Involved
Hi,

We have three 'tool boxes' to perform root cause analysis of quality issues. These are PDCA, 8D and Six Sigma toolbox. Where PDCA is the most simple and Six Sigma contains the most complex tools. I want to make a decision tree which could be used to determine which toolbox is the right choice for a particular issue.

Pls. see attached the first draft of the same. Any comments / ideas to improve the decision tree ?

( i know that there are almost always multiple causes for a problem but in order to keep the decision rule simple i want the engineer to judge if there is only one dominant root cause of the problem)

Thanks

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#### John Predmore

Trusted Information Resource
I prefer to think determination of root cause is partly in the eye of the beholder. I illustrate the idea with the example of 2 suppliers making the same part number. If supplier A parts sometimes arrive with rust on them, and supplier B parts never show any rust, everything else being equal, the customer can buy parts only from supplier B, a decision entirely within his control. As far as the customer is concerned, problem solved.

But supplier A, seeing he is about to lose his share of the business, has an incentive to discover the source of the rust. He may do an 8-D and discover the rusty parts only come from warehouse-1. So supplier B only stores these parts for this customer in warehouse-2. As far as the plant manager is concerned, problem solved.

But the warehouse-1 manager still has some other parts which sometimes show rust. It is worthwhile for him to do a Six Sigma study to understand where the condensation is coming from and to fix the technical problem. Three levels of the same problem, three different methods used, three different root causes identified. But it is the same problem scenario.

#### Bev D

##### Heretical Statistician
Super Moderator
Perhaps this is mere semantics - but I would quibble that these are not multiple root causes, rather causal steps along the way that enable the Problem to be dealt with. I used to go with the definition of root cause as that physical condition that when acted upon will prevent recurrence of the Problem. (I believe this is the definition used by Kepner-Tregoe)

In your example the cause of rust from warehouse 1 is that moisture from condensation is effecting the parts creating rust. Where does the condensation come from? That would be described as the root cause by KT.

However I have moved away from the term root cause as I have found it to be not very useful and often divisive. (some people will argue about anything to avoid taking action). I use the term causal mechanism* which is much broader in it's ability to describe and explain series of events and conditions that are at the 'root' of most problems.

To support your point I also describe and have utilize 8 different solutions** to Problems all of which are viable given time and circumstance:
1. Change the spec limits on The Y
2. Robust the Product against variation of the Y
3. Inspection Screening of The Y, including error proofing
4. Feedback Control: Measure Y to adjust a known X
5. Feed Forward Control: Measure X1 and Adjust X2
6. Robust the process against uncontrollable variation in The X
7. Implement a new process or design that eliminates The X
8. Reduce & control the variation of The X

*From The New Science of Fixing Things article Root Cause or Root Curse. Highly recommend it - a quick and insightful read.
**Some of these solutions are discussed as strategies to reduce output variation in:
“Strategies for Variability Reduction”, Quality Engineering, Volume 10, Issue 1, September 1997 , pp 125-136 by Stefan Steiner and Jock MacKay
A free version of this can be found here

#### Tahirawan77

##### Starting to get Involved
Thanks for your suggestions but if describe my situation i want train my quality team in various tools . The most simple tool is the PDCA cycle, It is the JUST DO IT approach. So if we have a simple problem and we need simple solution so we dont need to boil the ocean. The second set of toolbox is 8D and basically it contains the 7 basic QC tools. I want the Q team to use it when the solution is not already know. And lastly the Six Sigma GB toolbox contains the more advance statistical tools so if the root cause cannot be find using the first two tool box then we need to consider using the GB tools.

#### Jim Wynne

Thanks for your suggestions but if describe my situation i want train my quality team in various tools . The most simple tool is the PDCA cycle, It is the JUST DO IT approach. So if we have a simple problem and we need simple solution so we dont need to boil the ocean. The second set of toolbox is 8D and basically it contains the 7 basic QC tools. I want the Q team to use it when the solution is not already know. And lastly the Six Sigma GB toolbox contains the more advance statistical tools so if the root cause cannot be find using the first two tool box then we need to consider using the GB tools.
It seems like you might be trying to allow the tail to wag the dog.
• PDCA is not equivalent to "JUST DO IT," except for the fact that the "D" stands for
"Do." It's a strategy for solving any sort of problem; it applies to all three of the categories you've created.
• 8D is a reporting mechanism, not a strategy. It's used to tell what happened, not what should necessarily happen. It's applicable to use in any sort of problem-solving.
• "GB Tools" (Green Belt?) Solving any problem (or justifying decisions) might require statistical tools.
I understand what you're trying to do, but people who are trained in problem solving and root-cause analysis should be able to decide how to approach a given problem. Train your people well, and acquaint them with Occam's Razor.

#### Bev D

##### Heretical Statistician
Super Moderator
Tahirawan77 your flow diagram is fine. It shows what you have described. There is no need to change it.

What has my antenna vibrating is what appears to be your user standing of the ‘quality tool boxes’.

PDCA (actually PDSA; the S stands for Study which Shewhart (and Deming) intentionally chose because it is a deeper analysis than the common Check) is a recurring cycle that belongs in every problem solving effort, not just for teh simple implementations of ‘known’ solutions.

8D is actually a ‘framework’ or guide to what steps need to be taken to solve a problem whose cause is unknown. It is also - as Jim states - a ‘form’ to fill out to report what you did. Depending on your source, there are many ‘tools’ to use to accomplishing each step. The typical ‘GB’ tools are usually pretty disconnected and represent a mish mash of weak statistical tools. Unless your problems are fairly simple you should consider deeper investigation int strong problem solving methods….

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