The phone with no power
Your phone battery is empty by lunch. You buy a power bank and carry it everywhere. The phone is still empty by lunch every day.
You fixed the symptom, not the cause. The real cause was an app that ran all day. When you only treat the symptom, the problem comes back.
Key words
| Word | What it means | Example |
|---|---|---|
| Symptom | What you can see going wrong. | the app crashes |
| Root cause | The real reason a problem happens. | the memory is full |
| Root cause analysis | Looking for the real reason, not just the symptom. | asking why five times |
| Five whys | Ask why again and again until you reach the root cause. | why? why? why? |
| FMEA | A table that lists what could fail, how bad it is and how likely. | login could fail |
| Event tree | A picture that follows one event and what happens if each safety step works or fails. | server stops, backup starts |
| Escalate | Pass a problem to somebody with more skill or power. | tell a manager |
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Symptom or cause?
Have a go
Symptom or cause?
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.
Purpose and when to use it
Root cause analysis finds the real reason for a problem so you can stop it coming back.
Use it when a problem comes back, when it is serious, or when many people are affected. You do not need it for a single typing mistake.
Method 1: the five whys
Ask "why?" about the problem. Then ask "why?" about the answer. Keep going about five times. The last answer is usually the root cause.
Worked example: five whys on a bug
| Question | Answer |
|---|---|
| Why was the total in the app wrong? | The last item was not added. |
| Why was the last item not added? | The code stopped one item too early. |
| Why did the code stop early? | The programmer wrote the stop rule by hand and made a small mistake. |
| Why did nobody notice? | There was no test for a basket with only one item. |
| Why was there no test? | The team has no tests and no code review. This is the root cause. |
The cells students fill in are left blank in this preview.
Put the whys in order
Have a go
Put the whys in order
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.