topic Re: adjusting for selected multiple comparisons in SAS Procedures

adjusting for selected multiple comparisons

Ming — Thu, 06 Mar 2014 22:59:27 GMT

Hi,

I am trying to compare value from different time points to one specific time point for each individual. For example, for each individual, measurements were made every 15s starting from 30s to 3min (i.e. 11 measurements). I have 50 subjects in total. I want to compare all the different time measurements to the measurement at time 2:30 (ie. the 9th time point). If I perform a regular ANOVA with repeated measures with Tukey post-hoc, it will make adjustments for 55 comparisons (11x10/2) so everything will come out non-significant. Below is the code that I used...

data long;

input id sex$ @@;

do timept = 1 to 13;

input measurement @@;

output;

end;

proc mixed data=long;

class timept;

model measurement = timept;

repeated timept / subject=id;

lsmeans timept / adjust=tukey pdiff;

run;

However, what I wanted to compare are point 1 to 9, 2 to 9 3 to 9, .... 10 to 9, 11 to 9. So that's a total of 10 comparisons, not 55 comparisons (for complete comparisons). Is there a way to adjust for just 10 comparisons?

thanks for your help!

ming

Re: adjusting for selected multiple comparisons

PGStats — Fri, 07 Mar 2014 01:37:11 GMT

Try

lsmeans timept / adjust=Tukey pdiff=control('9');

Re: adjusting for selected multiple comparisons

Ming — Fri, 07 Mar 2014 18:18:24 GMT

Hi,

I got this error message and the output was the same as without the "pdiff=control('9')".

WARNING: Tukey's adjustment only applies to all pairwise differences, so these are taken.

do I need to rearrange my data differently?

thanks.

ming

Re: adjusting for selected multiple comparisons

Ming — Fri, 07 Mar 2014 18:35:57 GMT

Hi,

If I take out the option adjust=tukey so...

lsmeans timept / diff=control('9');

then I get an output that's what I want. However, I don't think this corrects for multiple comparison?

thanks.

ming

Re: adjusting for selected multiple comparisons

ballardw — Fri, 07 Mar 2014 18:53:24 GMT

I don't use proc mixed but if you want to completely ignore the other time frames exclude them from the analysis dataset for the procedure:

Proc mixed data=long (where= (timept <= 9))

might do what you're looking for.

Re: adjusting for selected multiple comparisons

PGStats — Sat, 08 Mar 2014 03:18:35 GMT

Use

ADJUST=DUNNETT

see section Comparing All Treatments to a Control in SAS/STAT(R) 9.3 User's Guide

Re: adjusting for selected multiple comparisons

SteveDenham — Mon, 10 Mar 2014 18:58:55 GMT

This is the best option for comparison to a single group. However, if someone wants to adjust for a limited number of comparisons (rather than all pairwise) and the comparisons are not all to a single group mean, the LSMESTIMATE statement in PROC GLIMMIX and PROC PLM (after using the STORE command in MIXED) is ideal.

Steve Denham