Subgroup Size and Chart
- April 7, 2018 at 1:36 pm #55969
If a lab analyzing a same reference standard on same machine everyday by different chemist and wanted to plot a chart, what subgroup can be choose and which chart is best to control the process? I-MR chart or x-bar-R or x-bar-s chart? I want to generate chart on monthly basis. thanksApril 9, 2018 at 6:09 am #202458
Generating a monthly chart isn’t really SPC. If you don’t respond to out of control results in short order, it’s just wallpaper typically.April 9, 2018 at 6:55 am #202459
I agree with @cseider – only I would add the adjective “expensive” to the word “wallpaper”.
As to you measurements – it does not appear that you have any obvious grouping of the data just individual data points from various chemists. Under those circumstances I would recommend an IMR chart with the proviso of identifying the individual chemists. What this would buy you would be a check on chemist-to-chemist variability. If everyone checks out the same – well and good and if they don’t you will know whom to address in the event one or more individuals have significantly worse variation.
Of course, on the positive side, in the event you have someone whose measurements are significantly better with respect to variation it would be worth understanding their approach to sample testing with the possibility that if everyone adopts that individuals methods there will be an overall improvement in measurement variation.April 9, 2018 at 7:54 am #202460
To my friend @rbutler
Yes, can you imagine a check on a standard showing a large deviation indicating too low of WBC detected on an analysis in a hospital. Wait until the end of the month to report/act on it.
I was wondering–if you privately could send some neat info on gage R&R’s in hospitals on blood testing? I have interesting conversations with doctors, even in my family, about how do they rely on the information or just “take another blood tube” if the results don’t look right? I am curious–since blood doesn’t stay good for long–how does standards be maintained for checking machine capability.April 11, 2018 at 5:04 pm #202466
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