Correlation Study between a new Measurement Device and an existing Device

#1
Hi all i have been assigned the task of introducing new measurement device to replace the current device which has been discontinued by the manufacturer. Gage R&R was carried out and results showed new device passed the acceptance criteria. But i have been asked to do correlation study between the new device and existing device before rolling out in production . my question is what is the best approach to this problem .
 
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Miner

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#2
There are a number of ways to do this. The Youden or ISO-plot as well as the Bland Altman plot.

See my blog, Intro to MSA of Continuous Data – Part 8: Comparison of Two Gages, for an overview.

There are a number of posts by Bev D on the Youden plot. See the following: Youden Plot. If I recall one of these posts has a job aid from Bev D.

There is also a Minitab macro for the Bland Altman plot available from Minitab.

I do not recommend performing a regression analysis as some may recommend.
 
#3
Thanks Miner for your reply and i have read your excellent blog . my question is why cant i use 2 sample t-test instead of youden plot obland altman plot .
 

Bev D

Heretical Statistician
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#4
Thanks Miner for your reply and i have read your excellent blog . my question is why cant i use 2 sample t-test instead of youden plot obland altman plot .
Because that test isn't intended for the type of data you have.
first you have matched pairs not grouped pairs.
second your data is NOT independent as it is repeated measures of the same things - which is a requirement for a 2 sample t test or standard correlation statistics for that matter.

Beyond the incorrectness of the statistical tool the answer can be wrong as a systemic bias between instruments will NOT be seen in a grouped analysis unless it is ridiculously large but will clearly be seen in a matched pair analysis (Youden and Bland-Altman)

See the attached spreadsheet for an example. There is a bias AND it passes a two sample t test. Note too that the individual instrument gage R&Rs would "pass".

The bias isn't huge, but the bland altman does properly quantify it so a rational decision can be made regarding it's impact.
 

Jim Wynne

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#6
ooops forgot the attachment!
:topic: I'm famous for doing that. One of the many things I like about Thunderbird, the Mozilla email client, is that if you use any form of the word "attach" or "attachment" in the body of your message, it pops up a reminder to actually attach something before the "send" button is hit.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#7
:topic: I'm famous for doing that. One of the many things I like about Thunderbird, the Mozilla email client, is that if you use any form of the word "attach" or "attachment" in the body of your message, it pops up a reminder to actually attach something before the "send" button is hit.
oooh! much better than the MS Exchange email thing...

but our IT police won't allow it. : (

you'd think these would be covered under ADA or at least AARP!
 
#8
thanks Bev i am having trouble in deriving any conclusions from this bland altman plot attached please can you give your valueable opinion .
 

Attachments

Miner

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#9
The first item of note is that there is a systematic bias of 0.16 between the two gages. You must decide whether this is of practical importance.

Here is a short article from MedCalc on the interpretation of the Bland Altman plot. Take note of example 1 for an example of proportional error. I would not call yours conclusive of an issue, but I would investigate further. Two items that catch my eye as worthy of investigation are the fact that the left half of the plot is above the mean line. Most of the second half straddles the mean line, but you have those three points outside the limits on the opposite side of the left half. I would evaluate more measurements in the -10 to -15 range.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#10
It would be helpful to also see the Youden plot vs the spec limits for each device and then for both devices...interpretation is aided when given complete context...
 
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