topic Re: Defining random statement for PROC GLIMMIX repeated measures in Statistical Procedures

Defining random statement for PROC GLIMMIX repeated measures

iatoloye — Thu, 06 Jan 2022 05:15:59 GMT

Hello All,

I am uncertain which of the statements I have for the random statement in my code is correct. I have a data set collected annually from the same set of plots and one treatment factor (with two levels) and three blocks. I am trying to determine the differences between the treatment levels using repeated measures.

Sample data

Treatment Block Year Yield @@;

0 1 2008 50

50 1 2008 75

0 2 2008 65

50 2 2008 85

0 1 2010 45

50 1 2010 65

...

Below are the codes:

First code:

proc glimmix data = Hist plots = studentpanel;
class block treatment Year ;
model lScore_Mean = treatment|year ;
random block block*treatment;
run;

Second code:

proc glimmix data = Hist.SHAPE plots=residualpanel;
class block treatment Year ;
model lScore_Mean = treatment|year / ddfm=kr2;
random intercept / subject=block ;
random year / residual subject=block*treatment;
covtest 'Is GLM OK?' glm;
run;

The results of the first code showed the treatment effect was not significant (p>0.1) while the results of the second code showed the treatment effect was significant (p<0.1).

I was hoping to get similar results but it seems I am not clear about the denominator degrees of freedom. I got different answers and I am not sure which one is right.

Thank you very much!😊

Re: Defining random statement for PROC GLIMMIX repeated measures

sbxkoenk — Sat, 08 Jan 2022 15:15:25 GMT

Hi,

I have moved your post to the "Statistical Procedures" board.

Re: Defining random statement for PROC GLIMMIX repeated measures

Rick_SAS — Sat, 08 Jan 2022 18:32:03 GMT

Can you clarify your experiment? The data seem to say that each block received Treatment=0 and Treatment=50 in the same year? Is that correct? I don't understand how the block can get both treatments at the same time.

Re: Defining random statement for PROC GLIMMIX repeated measures

iatoloye — Sat, 08 Jan 2022 21:26:34 GMT

Thank you for the question. I forgot to mention that the treatments 0 and 50 were applied only once in the same year, say the year 2000, to different plots. So we have 3 blocks and one treatment with 2 levels (0 and 50) (6 plots in all) and data collected from the same 6 plots annually (2000 till 2020).

Re: Defining random statement for PROC GLIMMIX repeated measures

iatoloye — Sat, 08 Jan 2022 21:29:03 GMT

I meant data collected biennially (2000 till 2020).

Re: Defining random statement for PROC GLIMMIX repeated measures

iatoloye — Sat, 08 Jan 2022 21:31:41 GMT

I have reposted the sample data

Treatment Block Year Yield @@;

0 1 2008 50

50 1 2008 75

0 2 2008 65

50 2 2008 85

0 3 2008 65

50 3 2008 85

0 1 2010 45

50 1 2010 65

0 2 2010 65

50 2 2010 85

0 3 2010 65

50 3 2010 85

Re: Defining random statement for PROC GLIMMIX repeated measures

Rick_SAS — Sat, 08 Jan 2022 23:40:06 GMT

This is a linear mixed model. Is there a reason you want to use GLIMMIX instead of PROC MIXED?

The documentation for PROC MIXED contains examples of block designs and repeated measures.

Re: Defining random statement for PROC GLIMMIX repeated measures

iatoloye — Sun, 09 Jan 2022 05:55:29 GMT

Thank you for the link to the documentation. There is no obvious reason for using GLIMMIX.

Re: Defining random statement for PROC GLIMMIX repeated measures

SteveDenham — Mon, 10 Jan 2022 15:24:23 GMT

You will find that GLIMMIX does some rather unusual things with denominator degrees of freedom, at least in my opinion (see the documentation - Details>Degrees of Freedom Methods for a full discussion). Check the output - in the Model Information table, it will indicate which method is used. That is most likely the source of the difference you are seeing.

SteveDenham