| These ideas come from statistical process control, which is a perspective that acknowledges two things: (a) All processes have some natural variation, and for as long as outputs fall in the range of natural process variation, we are looking at the same process. (b) Some processes apparently exhibit outputs outside of their natural variation. when this has happened something specific has occurred, and it is worth trying to find out what. In the second case, there are many possible reasons for exceptional outputs: - Measurement error, - Failure of the process, - Two interleaved processes masquerade as one, - A process improvement has permanently shifted the level of the output, - etc. SPC tells us that we should not waste effort on investigating natural variation, and should not make blind assumptions about exceptional variation. It says outliers are the most valuable signals we have, because they tell us we are not only looking at what we thought we were, but something ... else also. |