A formula to calculate critical values for Cochran’s C test can be found in:
R.U.E. ’t Lam, "Scrutiny of variance results for outliers: Cochran’s test optimized", Analytica Chimica Acta 659 (2010) 68–84. Equation 28 calculates the exact critical values for the traditional Cochran's C test. The formula works for any number of data sets, any number of replicates per data set, and at any confidence level. The equation requires critical F as input parameter. Critical F is obtained from Excel function FINV.
To make the contents of the article readily available, and to give further directions on how to perform this variance outlier test, I maintain a blog:
http://rtlam.blogspot.com/
Regards,
Ruben 't Lam
Quote:
Originally Posted by Dave Curtis[_2_]
Hi,
I'm trying to do some data analysis using Cochran's test for outlying
variances.
I have 4 replicate numbers from each of 20 laboratories. I calculate the
variance of each set of data.
I can work out the Cochran's test value by dividing the maximum variance by
the sum of all the variances.
Then I need to compare this with the Cochran critical values, which are
available from tables, but these have gaps, so I'd like to be able to
calculate them.
Does anyone know of a formula to calculate these?
Dave
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