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

In summary, three methods are used to measure form error of a round metal part. The base line is the traditional measuring method that is laborious but of the highest accuracy. A cnc machine measurement machine takes data with an analog tactile scan probe. Lastly, a very sophisticated digital 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.
  • #1
Ranger Mike
Science Advisor
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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
 
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  • #2
Correlation coefficient between what, confidence level of what?

A proper correlation coefficient shouldn‘t be expected to depend on the sample size.
 
  • #3
Ranger Mike said:
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.
 
Last edited:
  • #4
Ranger Mike said:
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.
 
  • #5
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
 

What is the importance of sample size in measuring Form Error of a round metal part?

The sample size is crucial in accurately measuring the Form Error of a round metal part because it determines the representativeness and reliability of the data. A larger sample size can provide a more precise estimation of the true Form Error of the entire population.

How do you determine the appropriate sample size for measuring Form Error of a round metal part?

The appropriate sample size can be determined by considering various factors such as the desired level of confidence, margin of error, and variability of the data. Statistical methods and sample size calculators can also be used to determine the minimum sample size required for a specific level of accuracy.

What is the recommended minimum sample size for measuring Form Error of a round metal part?

The recommended minimum sample size for measuring Form Error of a round metal part depends on the precision required and the level of confidence desired. Generally, a larger sample size is recommended for higher levels of precision and confidence. However, a sample size of at least 30 is often considered a good starting point.

What are the consequences of using an inadequate sample size for measuring Form Error of a round metal part?

If an inadequate sample size is used, the results may not accurately reflect the true Form Error of the entire population. This can lead to incorrect conclusions and decisions based on the data. It can also result in a waste of time, resources, and effort spent on collecting and analyzing data.

How can the sample size be adjusted if the initial results are not satisfactory?

If the initial results are not satisfactory, the sample size can be adjusted by increasing or decreasing the number of samples. However, this should be done carefully and only after considering the factors that affect sample size, such as the desired precision and confidence level. It is important to maintain consistency in the sample size throughout the study to ensure the validity of the results.

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