A rule of thumb is simply a general process used for a specific condition. This is different from rules, regulations, Standard Work, and other forms of documented instructions. In those cases, the required actions are specified. For a rule of thumb, there is no such formality.
An example of a rule of thumb might be “measure twice, cut once.” There is no real obligation to measure twice, but, over time, people have learned to follow this basic piece of wisdom.
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In your organization, rules of thumb can be dangerous. Because they are inconsistently administered, they deliver variable results. Rules of thumb certainly make life simpler, but when it comes to reducing defects, they are far too vague. They are, however, good indicators that a more formal process would help stabilize production, and would likely result in more predictable lead time and higher quality.