The Symptom Description must be complete and accurate. The more detailed the symptom description, the less work you'll need to do. If you need to call in a specialist, a complete and accurate symptom description will ensure the quickest and most accurate solution. A good symptom description minimizes the risk of "fixing the wrong problem", and helps determine the facts if there's a suspicion that you made the problem worse. Here is some of the information that goes into a quick and accurate symptom description:
However, that benefit must be weighed against the time and mental energy cost of obtaining this information and performing the differential analysis. Thus, for non-safety critical repairs with great access to test points, you'd probably devote only a few seconds to a couple minutes to these questions and their analysis. On the other hand, if diagnostic tests are difficult, unsafe or costly, it might be worth taking a day or more.
Distinction questions are double edged questions involving "is" and "is not", typically in the realms of "who", "which/what", "where", "when", and "to what extent". By developing a matrix of what is and what isn't, you can gain more focus on your hypotheses, hopefully arriving at a quicker solution.
This technique can be appropriate in Step 6 of the Universal Troubleshooting Process, Narrow it Down. It can be handy when you're stumped.
It's vitally important that whatever hypothesis you arrive at, no matter how iron clad it seems, be tested. To do otherwise invites second calls (recalls, bringbacks, etc.) and the problem getting out of the box.
Here are some examples of distinction questions:
Who has observed these symptoms, and who hasn't?
Which systems have these symptoms, and which don't?
Which subsystems malfunction, and which do not?
In what locations has this occurred and not occurred?
When did it occur, and when did it not?
How quickly does the symptom show after power up?
At what system age did the symptom occur, and at what age did it not?
How massive is the failure, and how massive or minor could it have been?
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