T
I have a question that I need cleared up for me. I'm not experienced in SPC at all. I have been to a few seminars and pretty much self taught alot of this material to myself. Here is a scenario I'm trying to figure out.
1. When I'm starting SPC for the first time and I gather say 100 data points. I enter them into my software and get a control chart that shows that I'm within statistical control. As data is entered on an hourly basis, the data points slowly start to shift and over time my average shifts with it indicating a fundamental process change is occurring (say tool wear). Now, my chart isn't going to visually show me an average shift is it? Yeah, i can read the actual numbers and tell that the average is shifting, but since my centerline is always readjusting with new data points, I don't visually see any shift such as line breaks at the centerline up or down.
What I want to know is if in order to see this shift, do you get your process average where you want it and then set the program to use only historical control limits and average. This way the control limits and average never change, but if you start to see data points making a trend upwards or downwards then that could indicate an average shift for which you would need to recalculate controls?
Hope this makes sense. Thanks.
1. When I'm starting SPC for the first time and I gather say 100 data points. I enter them into my software and get a control chart that shows that I'm within statistical control. As data is entered on an hourly basis, the data points slowly start to shift and over time my average shifts with it indicating a fundamental process change is occurring (say tool wear). Now, my chart isn't going to visually show me an average shift is it? Yeah, i can read the actual numbers and tell that the average is shifting, but since my centerline is always readjusting with new data points, I don't visually see any shift such as line breaks at the centerline up or down.
What I want to know is if in order to see this shift, do you get your process average where you want it and then set the program to use only historical control limits and average. This way the control limits and average never change, but if you start to see data points making a trend upwards or downwards then that could indicate an average shift for which you would need to recalculate controls?
Hope this makes sense. Thanks.