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How to proceed “Why-Why Analysis”

To identify the root cause for quick improvement

- Recurrence prevention of root cause -

October 1st, 2003

Yazaki CorporationToyota Business Unit

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• Under the Toyota Production System, “Why-Why Analysis” has an ironclad principle to repeat “Why” 5 times at Toyota, in order to identify the root cause of a trouble.

• This principle shows it is very important to identify the root cause and complete the recurrence prevention as quickly as possible when a certain trouble occurs.

• These days, “Why-Why Analysis” brings about some successful results in identifying the root cause of a quality problem and implementing recurrence prevention.

• I sometimes hear such complaints, “I don’t understand how to do Why-Why Analysis”, “I can’t repeat “Why” 5 times”, or “I can’t identify the root cause because I’ve found many possible causes”.

1. Introduction

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• For people in need, I give you tips to proceed “Why-Why Analysis” in this document.

I hope this document helps you understand and utilize “Why-Why Analysis”.

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(1) Clarify the content of a problem, and

understand the fact exactly.

　　　　 Clear up the problem with 5W1H.

　 When 　 ・・・ Date, Time

　 Where 　・・・ Place, Process

　 Who 　 ・・・ Person, Responsible person

　 What 　 ・・・ Component, Part

　 Why 　 ・・・ Reason, Cause

　 How 　 ･･･ Method, Means, Frequency, Condition

2. Before beginning Why-Why Analysis

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(2) Understand the structure (mechanism) and

function (role) of the problem.

Check drawings, QC process chart, Inspection specification, Operation standard etc. to completely understand the parts, characteristics, functions and structures.

When checking the defective products, it is necessary to keep such pictures and data as dimensional accuracy and measured characteristics etc.

(3) Check the problem part of defective products

and actual things of relevant parts.

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(4) Check the actual processing/assembly sites of

defective products or relevant parts.

On the day that the defective product is manufactured, ask the supervisors and operators if they’ve found some differences before/after the manufacturing, and record the findings.

If changes in 4M (Machine, Material, Method, Man) are found, record the details.

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If you ask “Why” only once, you cannot take a permanent measure for recurrence prevention because one-time “Why” provides superficial causes, ending up first aid.

　 There are numbers of examples that repeating “Why” 5 times has reached to the root cause, based on the fact of actual product and production site in an attempt to identify the root cause.

　 Followings are essential for the analysis.

3. 5-time Why

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① 　 Processing/Assembly conditions at the production site must be the same as those when the defect product was manufactured.

② 　 Processing and assembly must be checked right after processed or assembled.

It’s not too much to say that you can’t find out the root cause if the processing/assembly conditions are changed, because the existing situation at the production site is also changed.

This is a very effective condition to identify the root cause based on the fact.

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• Keep in mind that the more you are away from the time when a defective occurred, the more you have difficulty in identifying the root cause.

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Here is a model case of “Why-Why Analysis”

for the phenomenon:

With 5-time “why”, Toyota identified the root cause.

“Machine does not work”

4. Case study from Toyota Production System

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Case study: “Machine does not work”(1) “Why the machine stopped?” ⇒Because the fuse blew out due to the overload. (2) “Why the overload was applied?” ⇒Because the lubricant for the bearing was not enough. (3) “Why the lubricant was not enough?” ⇒Because the lubricant pump did not pump up enough. (4) “Why the pump did not pump up the lubricant enough?” ⇒Because the axis of pump had looseness resulted from

abrasion. (5) “Why the axis wore out?” ⇒Because some swarf came in resulted from the fact that

there was no filter.

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　 Repeating “why” 5 times, Toyota reached to the countermeasure:

Phenomenon (result)

1st cause

2nd cause

3rd cause

4th cause

5th cause

You must confirm based on the principles.

Machine stopped.

Fuse blew out due to

the overload.

Lubricant was not enough. Pump didn’t

pump up enough.

Axis wore out.Swarf came i

n.

“Install a filter”

As this example shows,

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• It is very important to stick to Genbutsu (actual product), Genba (actual site), and Genjitsu (fact) when you are trying to identify the root cause.

• If you come to deadlock, it is important to

- observe thoroughly.

- check if there are some problems with each item and activity.

- cling to questions you may have.

- repeat “Why” until you are fully satisfied.

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Let’s think about the following cases.

1) 1-time “why”• The supervisor replaced the fuse. This is a good me

asure as first aid. However, the same problem would reoccur in a couple of months because the measure is not taken for the root cause.

2) 3-time “why”• The lubricant pump could be replaced. However, th

e same problem would reoccur in a few months because this measure is not taken for the root cause as well. Also, the measure costs too much.

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3) 4-time “why” • The axis of pump could be replaced. However, the

same problem would reoccur in a few months because this measure is not taken for the root cause as well. The cost would be cheaper than that in the case of 2) above, but it takes long time to determine the frequency of replacement. 　

4) 5-time “why” • Permanent measure can be taken, and the machine

will never stop due to the same reason. Production problems come to the end when recurrence prevention for the root cause is completed.

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• In this way, if you pursue “why” 5 times, you can find out the root cause and can

take permanent measures.

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Now, if I repeat “why” further, followings come out.

(6) Why the filter was not installed?⇒Because the machine designer didn’t realize the necessity of the filter.

(7) Why the designer didn’t realize? ⇒ - Because he/she had no thoughts about swarf.

- Because there was no system to feed problems occurred at a plant back to the Equipment Design Department.

(8) Why the designer didn’t realize the treatment of swarf?⇒Because he/she didn’t care about issues other than the conditions for product processing.

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• In this case,

“Why-Why Analysis” is not focused on the relationship between “result and cause” with actual product and production site, but on the designer or system. In other words, this analysis is far away from the original purpose that is to take the recurrence prevention of root cause.

If it is necessary to improve the system, an appropriate department at the head quarter must be in charge and enhance the system.

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⑤ Do not take up an assumed cause which is not found from the fact.

① When analyzing, identify the cause from both aspects of “made the problem occur” and “made the problem flow out”.

② To figure out if you reach the root cause or not, confirm that the problem will never occur due to the same cause.

③ It is quite unusual that one phenomenon (Why) is caused by two causes at the same time.

④ Production problem should be solved by identifying root cause in production process errors but not as design errors.

5. Notices on “Why-Why Analysis”

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⑦ After finishing the analysis, read the report from the last “Why” to the first “phenomenon” with using the conjunction of “thus”, in order to confirm that the analysis makes sense, is reasonable, and does not have any leaps in logic.

Why⑤ Why④ Why① Phenomenon

Swarf came in.

Axis wore out. Fuse blew out.

⑥ Sentences to specify “phenomenon” or “Why” must be simple and short.

Thus,

Thus,

Thus, ・・・

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⑧ When continuing “Why”, stop analyzing a possible cause if you could determine, based on the investigation of actual products and production site, that a phenomenon is not caused by the possible cause.

⑨ Do not find causes in human mentality.

Ex: The operator was out of mind. The operator was tired.

⑩ Do not use the word of “bad” and unclear words in the sentences.

End