Programming the statistical procedures from SAS

Multiple comparisons against a control in the context of an heteroscedastic ANOVA model

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New Contributor
Posts: 3

Multiple comparisons against a control in the context of an heteroscedastic ANOVA model

Hi all,

 

I would like to have your input regarding a scenario where multiple pairwise comparisons against a control are conducted within the context of an Heteroscedastic Two-Way repeated measures ANOVA model.

 

Let's say I have 4 groups, group 1 is the control and I have multiple values taken for each group across time. Therefore, I would have an heteroscedastic ANOVA model with Group (4 levels), Time (let's say 6 levels) and their interaction as fixed factors.

 

The pairwise comparisons of interest would be Group 1 vs Group 2, Group 1 vs Group 3 and Group 1 vs Group 4.

 

My SAS mixed procedure would be as follows:

 

PROC MIXED Data= data METHOD=REML ;

  CLASS Group Time Subject ;

  MODEL Value = Group Time Group*Time / DDFM=KR ;

  REPEATED Time / GRP=Group Subject=Subject TYPE=CS ;/* this could be a different covariance structure */

  LSMEANS Group / PDIFF=Control("1") ADJUST= ??? ADJDFE=??? ;

RUN ;

 

I would greatly appreciate to know your input for the best approach to consider regarding the "???" that I have included in the above SAS code.

 

Thanks

Valued Guide
Valued Guide
Posts: 684

Re: Multiple comparisons against a control in the context of an heteroscedastic ANOVA model

Your code is OK. There are many choices for adjustments. I recommend adj=simulate. This works well for most problems, and gets around the assumptions underlying the other adjustment methods. For your application, you probably don't need a df adjustment. However, with the KR adjustment in the model statement, you could use adjdfe=row to take full advantage of the different df you may have for different means.

 

With this repeated measures, you probably also want to look into slices -- comparisons of means for one factor at each level of the 'other' factor.

 

Respected Advisor
Posts: 2,655

Re: Multiple comparisons against a control in the context of an heteroscedastic ANOVA model

I support @lvm's response here.  Your current LSMEANS statement compares differences in treatments averaged over all time points, and I am willing to wager upwards of 5 dollars US that you really want to compare differences in treatments at EACH time point.  That is what the SLICE and SLICEDIFF option are for.

 

Steve Denham

New Contributor
Posts: 3

Re: Multiple comparisons against a control in the context of an heteroscedastic ANOVA model

[ Edited ]

Hi @SteveDenham and @lvm,

 

Thank you both for your response. Please note that I had voluntarely remove the LSMEANS statement with the SLICE option in order to put the focus on the pairwise comparion's test performed across all timepoints.

 

Here is my full code:

 

PROC MIXED DATA=data METHOD=REML ;

  CLASS Group Time Subject ;

  MODEL Value = Group Time Group*Time / DDFM=KR ;

  REPEATED Time / GRP=Group Subject=Subject TYPE=CS ; /*this could be a different covariance structure */

  LSMEANS Group / PDIFF=Control("1") ADJUST=??? ADJDFE=??? ;

  LSMEANS Group*Time / PDIFF SLICE=Time ;

RUN ;

 

My main concern was regarding if the Dunnett test could be used in the context of an Heteroscedastic ANOVA model in order to compare all treated groups to a control group across all timepoints using the following statement:

 

LSMEANS Group / PDIFF=Control("1") ADJUST=Dunnett ADJDFE=ROW ;

 

Thanks

Respected Advisor
Posts: 2,655

Re: Multiple comparisons against a control in the context of an heteroscedastic ANOVA model

Well, it CAN, but that doesn't mean it is the best option.  ADJUST=simulate is by far the best option available in SAS for repeated measures analyses.

 

Steve Denham

New Contributor
Posts: 3

Re: Multiple comparisons against a control in the context of an heteroscedastic ANOVA model

Thank you Steve. Would you have any kind of documentation or paper that you could please refer to me regarding this adjustment?

 

Thanks

SAS Super FREQ
Posts: 3,548

Re: Multiple comparisons against a control in the context of an heteroscedastic ANOVA model

The advantage of the SIMULATE option is that it does not require as many assumptions about the data, and I suspect that is whySteve asserted that it is "the best."  I suspect that he meant that in practice simulation is very flexible and works for a wide range of data and models. If the data truly is a random sample from some distribution that is assumed by one of the other methods, then that method might give better results FOR THAT SAMPLE.  

Valued Guide
Valued Guide
Posts: 684

Re: Multiple comparisons against a control in the context of an heteroscedastic ANOVA model

I would cite P.H. Westfall, R.D. Tobias, and R.D. Wolfinger. 2011. Multiple Comparisons and Multiple Tests Using SAS, second edition. SAS Press.

 

The adjust=simulate is my favorite method.

 

SAS Super FREQ
Posts: 3,548

Re: Multiple comparisons against a control in the context of an heteroscedastic ANOVA model

And if you use ADJUST=SIMULATE(REPORT) , you can see useful information about the estimation.

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