I Help with sample size to measure Form Error of a round metal part

[Ranger Mike]

Hello all

First time on the Math side of this great Forum.

I need to do a correlation study.

There are 3 methods to measure Form Error of a round metal part.

1. The base line is the traditional measuring method that is laborious but of the highest accuracy.
2. A cnc machine measurement machine takes data with an analog tactile scan probe
3. Lastly, a very sophisticated digiatal tactile scan probe.

Previous observations between the three methods have variacen of ½ to 1 micron.I am seeking a very strong correlation coefficent and high confidence level.

What would be a good sample size?

Thank you

 

[mfb]

Correlation coefficient between what, confidence level of what?

A proper correlation coefficient shouldn‘t be expected to depend on the sample size.

 

[WWGD]

Hello all

First time on the Math side of this great Forum.

I need to do a correlation study.

There are 3 methods to measure Form Error of a round metal part.

1. The base line is the traditional measuring method that is laborious but of the highest accuracy.
2. A cnc machine measurement machine takes data with an analog tactile scan probe
3. Lastly, a very sophisticated digiatal tactile scan probe.

Previous observations between the three methods have variacen of ½ to 1 micron.I am seeking a very strong correlation coefficent and high confidence level.

What would be a good sample size?

Thank you

If you want to test for the existence of a linear relationship, the standard statistic is the t statistic ( Df=n-2) for a sample of size $n$ and sample correlation $r$, is given by

$\frac {r \sqrt{n-2}}{1-r^2}$

EDIT: This is to test the hypothesis on whether there is no linear relationship as null , versus there is one, at your choice of confidence level.

 

[Dale]

I am seeking a very strong correlation coefficent and high confidence level.

Although correlation is often done for such measurements it is not a very good thing to do statistically. You should read the famous paper by Bland and Altman.

 

[Ranger Mike]

thank you fro the timely replies. Having spent all day revisiting the old calc book i am back up to speed on stats. Granted one correlation does not a decision but i think it will provide a lot of insight into our current metrology capabilities.
I have decided 61 sample size will be a good base line for 95% confidence per T table and my non scholastic customers like lots of samples to compare. I suspect the reading from all CMM and rotary instrument will be within a 1 micron range and i am betting on it being within 1/2 micron from past experience. Back in 1980 it was 2 1/2 micron!
would my data be of any benefit you the great people on this forum?
thank you
rm