Section 21. Fishbone Diagram


21. Fishbone Diagram

Overview

The ubiquitous Fishbone Diagram (also known as a Cause & Effect Diagram) is a well-known quality tool but was dropped from the Six Sigma Process Improvement Roadmap a number of years ago, replaced completely by the Process Variables Map and Cause & Effect Matrix combination. It does however have a place in a few Lean Sigma problem categories that aren't related to a single process.

The Fishbone Diagram (obviously named for its looks) is a Team brainstorming tool to help identify potential root causes to problems, or in Lean Sigma terms Key Process Input Variables (KPIVs) or Xs. The problem is tackled by examining six major process related areas (Branches):

  • Man (People)

  • Machine

  • Material

  • Measurement

  • Method

  • Mother Nature (Environment)

These are then broken down further into Sub-Branches, effectively following the 5 Whys principal until some potential root causes (Xs) are identified. Therefore, the tool is another means to generate process Xs.

Figure 7.21.1. Structure of a Fishbone Diagram.


An example Fishbone Diagram is shown in Figure 7.21.2.

Figure 7.21.2. Example Fishbone Diagram relating to warehouse inventory variances.


Logistics

The Fishbone Diagram is absolutely a Team endeavor. It typically takes about one to two hours to complete in a brainstorming session with Team members and other key process experts. It is best done in one sitting, so if it can't be completed during the time available postpone until a more suitable time.

Belts are strongly advised not to create a straw man of the Diagram prior to the meeting; this approach usually causes a lack of buy-in to the result and also potential missed Xs.

Roadmap

The Roadmap is relatively straightforward:

Step 1.

Create the Fishbone structure on a flipchart-sized sheet of blank paper. Place on the wall in landscape orientation. Draw a horizontal line with a box connected at the far right and write the problem or effect in the box. Draw six branches off the main stem and categorize them as People, Material, Method, Machine, Measurement, and Environment.

Step 2.

Brainstorm Xs in each of the categories and capture them onto a branch or sub-branch under the correct category. It is often useful to use a sticky note for each X so they can be moved around freely. Use the 5 Whys to drill down each cause category until an X is reached. Similar to the Process Variables Map, Xs need to be actionable or tangible items. For example, an X "Operator" isn't really an X, it is more of a group of Xswhat specifically is it about the Operator?:

  • Height

  • Weight

  • Handedness

  • Availability

  • Hair color

Table 7.21.1 can help break down the groups and also identify those trickier Xs.

Table 7.21.1. Fishbone Diagram Checklist[39]

Measurement

Methods

Machine

Poor repeatability

Incorrect definition

Machine maintenance or calibration

Poor reproducibility

Incorrect sequence

Machine controls or lack of controls

Poor accuracy

Missing definitions, implicit rules

Machine fault or defect

Poor stability

Poor process controls

Software or network fault

Poor linearity

Poor measurement controls

Machine related contamination

Invalid measurement or test method

Lack of critical information

Machine tooling or fixtures

 

Incorrect information

Incorrect machine or tester

Excessive test or measurement

Excessive queues or outtime

 
 

Handling

 
 

Orientation

 
 

Poor management of change

 
 

Incorrect revision

 

Materials

Environment

People

Defective

Physical environment (temperature, lighting)

Level of staffing

Off-specification

 

Training

Contaminated

Security or safety systems

Competency or experience

Improper storage conditions

Distractions in the environment

Supervision

Labeling or identification

Particulates

Conflicting goals

Incorrect amount or quantity

Contamination

Compliance with procedures

Improper transportation or handling

 

Personality issues

Expiration date exceeded or unknown

 

Cognitive ability and function

  

Knowledge deficit

Problem with product design

 

Communication with peers or supervisor

Wrong materials

  


[39] From SBTI's Lean training material.

Interpreting the Output

The biggest mistake Belts make is reverting to the traditional use of the Fishbone Diagram; Process Improvement Teams would look at all the Xs generated and from gut-feel decide which Xs to pursue. This is incorrect in Lean Sigma and in fact, could completely derail the project.

The Fishbone Diagram is used only to identify the potential Xs, not to prioritize them. Subsequently, more suitable tools (the Cause & Effect Matrix, Failure Mode and Effects Analysis, and Multi-Vari Study) narrow them down to the critical few. There should be absolutely no action items coming from the Fishbone Diagram other than to transfer the Xs to the next tool.




Lean Sigma(c) A Practitionaer's Guide
Lean Sigma: A Practitioners Guide
ISBN: 0132390787
EAN: 2147483647
Year: 2006
Pages: 138

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