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.