SPC Charts With Slow Moving Data

Discussion created by rdube02 Champion on Oct 9, 2012
Latest reply on Oct 11, 2012 by mhamel

I am trying to develop a Processbook display that shows a variety of data points from a machine. I'm using X-Bar charts, taking 90 samples before end time, and each sample has a 0:05 calculation period and a 0:02 sample period with a sample size of 2.


My goal is with 90 samples and 0:05 calculation period, my charts are all approximately a 450 second slice of time.


For data points that change very little over a long period of time, I get a straight line, but if there is any sudden change in value, instead of drawing a line from the last data point to the new, lower or higher value, it shifts the entire line down or up.


I think this occurs when the change is very small, for example a change from 0.255 to 0.251. My SPC chart is set up with a range that can display such changes, but instead of charting them it's shifting the entire line up and down - making the previous datapoints inaccurate.


Could this be somehow related to the fact that the value isn't changing enough to produce an actual "raw" PI data point due to compression? Is it a bug in SPC charts?


Thanks for any insight you can offer.