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st: Limits to df in invnchi2


From   Marcello Pagano <Pagano@hsph.harvard.edu>
To   statalist@hsphsun2.harvard.edu
Subject   st: Limits to df in invnchi2
Date   Fri, 12 Sep 2003 16:52:37 -0400

Jeffrey Simons(jsimons@usd.edu) is having trouble posting this:

From Stata manual:

invnchi2(n,L,p) returns the inverse cumulative noncentral chi-squared
distribution. If nchi2(n,L,x) = p, then invnchi2(n,L,p) = x (n must be an integer,1<=n<=200, 0<=L<=1000).

Why does n (the degrees of freedom) need to be <=200?

I am trying to utilize MacCallum et al., (1996) formulas for determining
sample size for covariance structural models. He provides equations and SAS code at the end of the article and it involves the above chi2 distributions.

I wrote a program to do this in Stata but naturally, the model I want to
test has 246 degrees of freedom. Why is there this limit?

Thank you
--
______________________________________________________________________

Marcello Pagano
Biostatistics Department Tel: 1-617-432-4911
Harvard School of Public Health Fax: 1-617-739-1781
655 Huntington Avenue email:pagano@biostat.harvard.edu
Boston, MA 02115 http://biosun1.harvard.edu/~bio200
USA

eppur si muove


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