When problem solving, you want to be mindful of tacit assumptions. These are the things that are considered to be true and often go unquestioned.
An example;
A manufacturing company was tolerating a 3% failure rate at final test. The failures appeared at random and this made it difficult to identify a root cause. There were several attempts at solving the problem but none were successful.
A few months later, the company was implementing a Six Sigma training program and one of the students took on a project of collecting data on this very problem. The task was to find data that may correlate with failures at final testing. At an upstream process, all products were heated for 12 hours at 400 ºF. The traceability records only revealed the oven ID but not the location within the oven. For perspective,
- a hundred products fit into a tray
- each shelf carried four trays
- each oven had 12 shelves
The student identified the tray ID, shelf number and position (front-left, back-right, etc) for each tray. This was done for just a few oven loads to see if the data would be useful for the investigation.
She discovered that 93% of all test failures were heated on the bottom three shelves of the oven!
This was a real 'a-ha' moment for the company. A dozen or more ideas about the root cause had promptly converged to one.
A follow-up investigation had a technician load a few thermocouples inside the oven at various positions. It was revealed that the bottom of the oven was around 25º cooler than the top.
Why was this not detected earlier? Everyone assumed that the oven was operating at the right temperature. The one thermocouple that was built-in to the oven was measuring temperature near the top.
Always be mindful of your assumptions. They won't always be correct.
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