Re: Two gauges - Two measurements correlation I do not know how to explain
Hi,
Thank you very much for your reply. I like the attachments.
I summarised what your suggestion as follow,
1. Do repeatability for each gauge first to understand the repeatability of each gauge is good before compare them.
2. Plot "Youden plot" to understand the bias and use "Bland -Altman" to understand the bias magnitude.
I have some further questions,
1. To do the repeatability for a gauge, I need to sample a full process variation. What about the data for "Youden plot" and "Bland - Altman"?
2. Our company's product is glass sheet and we are planning to buy a laser cutter to cut the glass sheet to the right dimension. After the cutter, we would like to install a gauge, called Dimension Gauge, to measure glass sheet dimension including X, Y and squareness. Our cutting dimension spec. is 2500.30 +/- 0.10mm. Here are the questions
a) As you mentioned, we have to do repeatability with 30 samples which cover the full process variation for our gauge. It means that I have to collect samples which dimension is from 2500.20~2500.40. Is this correct?
b) The laser cutter and Dimension Gauge are new for our process. Can I qualify them together? What I mean is that set the cutting size to 2500.20 for laser cutter and cut the glass sheet, and then measure the glass sheet by the Dimension Gauge. Repeat the same process, set the laser cutter to 2500.21~2500.40, until we collect 30 samples. Or should I qualify one by one?
3. Our Dimension Gauge vendor proposes that the gauge has 30um repeatability. Can I use the way you suggested to qualify this gauge?
4. People in this fourm told me that I have to calculate the gauge measurement uncertainty, is it the same as yours?
5. In my company, we use GRR to qualify a new gauge, here is the way we do, collect 30 samples which cover the product tolerance and calculate the GRR by Minitab, and if the % Total Variation is < 10%, then this gauge is acceptable. What is the difference between our GRR and your suggestion, repeatability?
6. I put L1 and L2 data in the column on A and B in "Discrimination Plot" spread sheet, however, I do not see the Measurement Discrimination Plot shows points. Did I make mistakes? Please see the attachment.
P.S: L1, L2, L3 and L4 refer to our glass dimensions.
Regards,
A quick look at the scatter diagrams (aka Youden Plots) shows that you don't have much variation in teh parts you measured vs the measurement error this can lead to misinterpretation of any statistical test.
A few points: correlation and regression are not appropriate statistical tests for measurement error. (You can use them but they must be interpreted with caution as they are not designed to assess measurement error directly.)
The intraclass correlation coefficient was designed to assess measurement error; the Youden plot was designed to display measurement error including comparisons of two or more gauges; the Bland -Altman and paired t tests are also applicable for method comparison.
I have attached a document briefly describing each of these as well as a spreadsheet that perfroms the calculations and plots the data.
Did you assess the repeatability of the individual guages first? without knwoing if the guages themselves are repeatable, any analysis of gauge to gauge performance is tenous at best.
can you describe what L1, L2 L3 and L4 refer to?
did your samples span the full range of known variation? what are the spec limits?