G matrix in SAS Proc Mixed [General Sta­tis­tics]

posted by d_labes  – Berlin, Germany, 2008-09-22 13:35 (5638 d 09:41 ago) – Posting: # 2381
Views: 14,871

Dear Sathya,

❝ How can I find rho by using SAS?

❝ And how can I get G matrix in SAS? I tried with G and Gcorr. But I am

❝ unable to get G (1, 1), G (2, 2) and G (1, 2).


Look at your SAS output of Proc MIXED again. If you use the G option you will have a piece of output named "Estimated G Matrix". G(i,j) is the element with row i and column j.
For instance:
 Row    Effect       treat.    Subject        Col1       Col2
   1    treatment    R           1          0.1181     0.1058
   2    treatment    T           1          0.1058     0.09486

Here G(1,1) is 0.1181, G(1,2) is 0.1058 and so on.
Yes, its that easy :cool: with matrices. Try to remember your math courses on that! They are not the same thing as Matratzen (German word, in english mattresses).

Why do you need rho?
If you are interested in, take the formulas above in this thread, a little puzzled math and you have a formula for rho from the elements of the G matrix. I left this for your excercise.
The option GCORR gives you a value of rho in the off-diagonal element of the "Estimated G Correlation Matrix".
Or parameterize with the CSH structure instead of FA0(2) (look at your SAS/Stat manual) and you will get directly an estimate of rho under "Covariance Parameter Estimates". Its name is CSH in the SAS output.

For the rest of your questions:
They look like the question "How can I earn a lot of money?". How shall I answer this? :ponder:
Have a look at the Guidance of the country you will submit to. There are in most cases enough hints what you have to submit (up to such very special and detailed wishes of the regulators in the new EMEA draft 'The presentation of the findings of a bioequivalence trial should include a 2x2-table that presents for each sequence (in rows) and each period (in columns) means, standard deviations and number of observations for the observations in the respective period of a sequence.').

Concerning PBE/IBE search the forum and look again into Guidances.
You will find that usually no regulatory body will request this.

Regards,

Detlew

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