I am working with a client who has experienced an increase in variability in the thickness of their product. I was able to get my hands on just over a year's worth of recorded data and the plot was quite revealing. the chart is attached - please ponder it for a moment.
Their process has some sources of variation between batches but look at how the data points appear after June 2008 (middle of the graph). At this point in time, someone replaced the measuring tool. The older digital indicator was able to provide five decimal places whereas the new digital indicator provided only three. This shows up as an increase in the short term variability in the data.
The measurement system is one of the sources of variability in a data set and in many cases, it is the largest source. It is the easiest to correct and easy to prevent.
I took the original data set and plotted the differences between each data point. Here, it is clear that the new tool increases the variability of the measurement system.
The wrong tool can be very costly - rejecting good product just because of measurement error.
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