SC QUALITY GUILDFishbone and 5 whys
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Brainstorm causes on a fishbone, pick the most likely, then ask why until you reach something you can act on, writing down the evidence at every step.
ON THE EXAM · CSSYB IV.B, I.E.1 · FREE · NOTHING YOU TYPE LEAVES YOUR BROWSER
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Two tools, one job
The fishbone (cause-and-effect or Ishikawa diagram) is for breadth: getting every plausible cause out of the team's heads and into categories, so nothing obvious is missed. The 5 whys is for depth: taking one cause and following it down until you reach something the process can change. Used together, the fishbone stops the whys from chasing the first idea mentioned, and the whys stop the fishbone from ending as a poster of guesses.
The categories
The six categories here are the common manufacturing set: people, machine, method, material, measurement and environment, often called the 6Ms. They are a prompt, not a rule. Service processes often use policies, procedures, people, place and systems instead. Rename them to whatever makes your team think of causes it would otherwise miss.
Do not leave measurement out. A surprising number of "process" problems turn out to be a gauge that cannot tell good from bad.
Five is not the number
The name is a rule of thumb. Some chains reach an actionable cause in three whys; some take seven. Stop when the answer is something the process can change and you can test. Stop sooner if the next answer would be outside anyone's control.
The column that matters most is evidence. A chain of whys answered from opinion feels convincing and is often wrong, because each answer is the most plausible-sounding one, not the true one. Write down what shows each answer is true: a measurement, an observation, a document.
"Human error" is where the analysis starts
If the chain ends at "operator error", ask two more questions: why did the process make the error possible, and why did nothing catch it? The tool flags root causes that blame people for this reason.
A root cause is a hypothesis until you test it
The strongest test is to switch the effect on and off: remove the cause and the problem goes away, put it back and the problem returns. Write down how you will check before you act.
On the Yellow Belt exam
Root cause analysis is topic IV.B of the 2022 CSSYB Body of Knowledge, and the 5 whys is one of its named methods. The cause-and-effect (fishbone) diagram is one of the seven basic quality tools in I.E.1. Expect to be asked which tool suits a situation, and which category a listed cause belongs in.