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Yes we do break down the complaints to Pareto the causes.

Lets add another slant. I am sure those of you that work in companies that use the type of complaints monitoring I describe also perhaps have a target number of complaints, possibly set by upper/corporate management. Should we use this value given as our upper specification limit, and zero as our lower specification limit, and then plot our monthly count and determine our mean (which should be half way between zero and our complaints target) and our process capability (which may be +/- 6 standard deviations within the specification limits) ?

I just think that this is much more meaningful as a simple trend line on such a graph will always be either up or down, and will be randomly up or down depending on where the first count of complaints on our graph came within our normal distribution. i.e. if we plot a rolling twelve months, and on the first month we had a count of complaints that was towards the upper specification limit, then for the rest of the year (assuming the process is in control) the trend line will be downward. If however our count on the first month is towards the bottom specification limit (zero) are trend line will always be upwards.

As such the trend line on such a graph actually tells us nothing about whether the process performance is getting better or not. What would tell us that is if the mean value moved towards the lower specification limit, and/or the value of standard deviation (variance) reduced.