Greek letters could be used as placeholder in every way an author likes them to put, so there are only very few greek letters which have a common meaning (like mu and sigma). If you're interested in the meaning of your theta, you have to give the reference for the article or book where you found it.

There is an article about the Clements-method written by John Clement himself, published in Quality Progress in september 1989. The title is "Process Capability Calculations for Non-Normal Distributions". The

archive for past issues of QP go only back until 1995, but maybe you could find a university nearby which has older issues as hardcopies.

But the theta you mentioned is not included in this article. (Skewness and curtosis are used, but with different notations than theta.)

Theta isn't mentioned in the standard werk for distributions also, there are betas and kappas in the section about Pearson curves (Johnson, Norman; Kotz, Samuel; Balakrishnan, N. [1994]: Continuous Univariate Distributions: Volume 1. John Wiley & Sons, 2nd edition, ISBN 978-0471584957, p.15ff.)

Besides I agree with Bev: The Clements method is a nice method if you want to do research in a difficult mathematical area, but not applicable for normal process data.

Regards,

Barbara