From: Keith Wong <keithw_at_med.usyd.edu.au>

Date: Sun 04 Jul 2004 - 17:21:36 EST

R-help@stat.math.ethz.ch mailing list

https://www.stat.math.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Sun Jul 04 17:28:32 2004

Date: Sun 04 Jul 2004 - 17:21:36 EST

> g2 = lme(Y ~ time + group + time:group + W + Z, random = ~ 1 | id, data =

datamod)

numDF denDF F-value p-value (Intercept) 1 42 5.54545 0.0233 time 4 42 16.41069 <.0001 group 1 11 0.83186 0.3813 W 1 42 0.07555 0.7848 Z 1 42 45.23577 <.0001 time:group 4 42 3.04313 0.0273

I compared the results using SAS proc mixed:
proc mixed data = datamod;

class id time group;

model Y = time group time*group W Z /s;

random int / sub = id;

run;

And get the following anova table for the fixed effects:

Type 3 Tests of Fixed Effects Num Den Effect DF DF F Value Pr > F time 4 42 2.55 0.0534 group 1 42 0.54 0.4664 time*group 4 42 3.04 0.0273 W 1 42 8.80 0.0050 Z 1 42 32.52 <.0001

I am perplexed to see that the test for the main effect “time” is quite different. Both models seem to be specified equivalently to me, am I doing something wrong – particularly with the inclusion of the time-dependent covariates W and Z? I have looked at the data in both programmes and they are the same. There are no missing observations.

I a simpler model without the time-dependent covariates, and in this case the
results are similar:

[R]

> g1 = lme(Y ~ time + group + time:group, random = ~ 1 | id, data = datamod)

> anova(g1)

numDF denDF F-value p-value (Intercept) 1 44 3.387117 0.0725 time 4 44 10.620547 <.0001 group 1 11 0.508092 0.4908 time:group 4 44 3.961726 0.0079

**[SAS]
**

proc mixed data = datamod;

class id time group;

model Y = time group time*group /s;

random int / sub = id;

run;

Type 3 Tests of Fixed Effects Num Den Effect DF DF F Value Pr > F time 4 44 7.75 <.0001 group 1 44 0.51 0.4797 time*group 4 44 3.96 0.0079

Secondly, is there no R equivalent of the “LSMEANS” statement in SAS? Is there a work-around?

I would very much appreciate assistance.

Thanks. Keith

R-help@stat.math.ethz.ch mailing list

https://www.stat.math.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Sun Jul 04 17:28:32 2004

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