topic Re: Question About Default Means Separation in PROC GLIMMIX in Statistical Procedures

Question About Default Means Separation in PROC GLIMMIX

jbibb14 — Fri, 06 Dec 2024 03:47:30 GMT

Hello,

I am trying to figure out what the default separation method is when I run this PROC Glimmix code. Usually I specify it to run Tukey, but when I remove that specification from my code I wanted to know what method is the default that runs?

Code Example Below:

proc glimmix data=crapes;
class trt rep color;
model arl = color trt color*trt;
random rep;
lsmeans color trt color*trt/diff lines;
run;
quit;

Typically in the lsmeans line I have adjust=tukey, but as I stated I am curious as to what program runs the means separation when that statement is removed and it runs as typed above.

TIA!

Re: Question About Default Means Separation in PROC GLIMMIX

StatDave — Fri, 06 Dec 2024 04:03:31 GMT

This is covered in the description of the DIFF option in the documentation of the LSMEANS statement. By default, the DIFF option produces all possible pairwise comparisons of the LS-means without any adjustment using simple t-tests. You can specify optional values after DIFF= to request differences with the average LS-mean or differences with a control level. If you want mean comparisons to be adjusted, specify the ADJUST= option with one of the available methods.

Re: Question About Default Means Separation in PROC GLIMMIX

jbibb14 — Fri, 06 Dec 2024 04:10:25 GMT

I am still new to running everything with this software, in your opinion would there ever be a reason to run the code i typed without entering a diff or adjust parameter?

Re: Question About Default Means Separation in PROC GLIMMIX

PaigeMiller — Fri, 06 Dec 2024 10:50:23 GMT

@jbibb14 wrote:

I am still new to running everything with this software, in your opinion would there ever be a reason to run the code i typed without entering a diff or adjust parameter?

Yes, of course you can run the program exactly as you wrote it, without other options. You can use different statistical methods by specifying other options, if you want. There are probably a dozen different ways to compare means that are useful in some situations, as explained in the documentation.

Re: Question About Default Means Separation in PROC GLIMMIX

SteveDenham — Tue, 10 Dec 2024 18:48:18 GMT

Would there ever be a reason to not include DIFF=method>? Absolutely yes. All of the methods are designed to control type I error (false positives). However, suppose that it is far more important to control type ii error (false negative rate) in whatever field you are working. Then the unadjusted values give a greater ability to declare a difference significant. As an example, suppose you are working on a safety study for a new drug. In that study, it would be much more important to detect a true difference to protect you from declaring the drug "safe" when in fact it causes some change that may indicate a potential safety concern.

SteveDenham