During my Engineering days we would occasionally run into physical phenomena that didn't have a Gaussian basis. Some of these were lengths of lightening strokes, she speed of sound in water, etc., etc.

Specifying a few sigma as a measure of reliability, can occasionally represent unreliability when the underlying physical parameters are not symmetrical. This can lead to misleading results. I have read that the scientists at CERN generally use a 5-sigma threshold for high reliability determination of physical parameters.

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