Qc Tools – Fishbone Diagram
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Quality pros have many names for these seven basic tools of quality, first emphasized by Kaoru Ishikawa, a professor of engineering at Tokyo University and the father of “quality circles.”
Start your quality journey by mastering these tools, and youll have a name for them too: “indispensable.”
Cause-and-effect diagram (also called Ishikawa or fishbone chart): Identifies many possible causes for an effect or problem and sorts ideas into useful categories.
Check sheet: A structured, prepared form for collecting and analyzing data; a generic tool that can be adapted for a wide variety of purposes.
Control charts: Graphs used to study how a process changes over time.
Histogram: The most commonly used graph for showing frequency distributions, or how often each different value in a set of data occurs.
Pareto chart: Shows on a bar graph which factors are more significant.
Scatter diagram: Graphs pairs of numerical data, one variable on each axis, to look for a relationship.
Stratification: A technique that separates data gathered from a variety of sources so that patterns can be seen (some lists replace “stratification” with “flowchart” or “run chart”).
Fishbone Diagram
Also Called: Cause-and-Effect Diagram, Ishikawa Diagram
Variations: cause enumeration diagram, process fishbone, time-delay fishbone, CEDAC (cause-and-effect diagram with the addition of cards), desired-result fishbone, reverse fishbone diagram
Description
The fishbone diagram identifies many possible causes for an effect or problem. It can be used to structure a brainstorming session. It immediately sorts ideas into useful categories.
When to Use a Fishbone Diagram
When identifying possible causes for a problem.
Especially when a teams thinking tends to fall into ruts.
Fishbone Diagram Procedure
Materials needed: flipchart or whiteboard, marking pens.
Agree on a problem statement (effect). Write it at the center right of the flipchart or whiteboard. Draw a box around it and draw a horizontal arrow running to it.
Brainstorm the major categories of causes of the problem. If this is difficult use generic headings:
Methods
Machines (equipment)
People (manpower)
Materials
Measurement
Environment
Write the categories of causes as branches from the main arrow.
Brainstorm all the possible causes of the problem. Ask: “Why does this happen?” As each idea is given, the facilitator writes it as a branch from the appropriate category. Causes can be written in several places if they relate to several categories.
Again ask “why does this happen?” about each cause. Write sub-causes branching off the causes. Continue to ask “Why?” and generate deeper levels of causes. Layers of branches indicate causal relationships.
When the group runs out of ideas, focus attention to places on the chart where ideas are few.
Fishbone Diagram Example
This fishbone diagram was drawn by a manufacturing team to try to understand the source of periodic iron contamination. The team used the six generic headings to prompt ideas. Layers of branches show thorough thinking about the causes of the problem.
Fishbone Diagram Example
For example, under the heading “Machines,” the idea “materials of construction” shows four kinds of equipment and then several specific machine numbers.
Note that some ideas appear in two different places. “Calibration” shows up under “Methods” as a factor in the analytical procedure, and also under “Measurement” as a cause of lab error. “Iron tools” can be considered a “Methods” problem when taking samples or a “Manpower” problem with maintenance personnel.
Excerpted from Nancy R. Tagues The Quality Toolbox, Second Edition, ASQ Quality Press, 2004, pages 247-249
Check Sheet
Also called: defect concentration diagram
Description
A check sheet is a structured, prepared form for collecting and analyzing data. This is a generic tool that can be adapted for a wide variety of purposes.
When to Use a Check Sheet
When data can be observed and collected repeatedly by the same person or at the same location.
When collecting data on the frequency or patterns of events, problems, defects, defect location, defect causes, etc.
When collecting data from a production process.
Check Sheet Procedure
Decide what event or problem will be observed. Develop operational definitions.
Decide when data will be collected and for how long.
Design the form. Set it up so that data can be recorded simply by making check marks or Xs or similar symbols and so that data do not have to be recopied for analysis.
Label all spaces on the form.
Test the check sheet for a short trial period to be sure it collects the appropriate data and is easy to use.
Each time the targeted event or problem occurs, record data on the check sheet.
Check Sheet Example
The figure below shows a check sheet used to collect data on telephone interruptions. The tick marks were added as data was collected over several weeks.
Check Sheet Example
Excerpted from Nancy R. Tagues The Quality Toolbox,