R help - Jan 2011 - Linear mixed model: question about t-values

Dear all,

I have a question about the output of linear mixed model fitted in R
using nlme package. In particular, what are the t-values that are given
in an output, how are they calculated and based on what test? I guess it
cannot be a simple Student t-test, otherwise how can the simple Student
t-test test for significance of interactions, right? I cannot find this
information in any of R help resources on linear-mixed models and I also
checked few books.

Example: the part of R output

 model.b<-lme(diff~age+height,random=~1|field/replicate)

 Linear mixed-effects model fit by REML

Fixed effects: diff ~ age + height_cm 
                Value Std.Error  DF   t-value p-value
(Intercept) 172.83559 27.094642 107  6.378958  0.0000
ageyoung     -5.28206 11.239981   4 -0.469935  0.6629
height_cm    -0.67662  0.450183 107 -1.502997  0.1358

Thank you very much for your help!

Best wishes,
Olja


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Kostenko, Olga <O.Kostenko <at> nioo.knaw.nl> writes:
> I have a question about the output of linear mixed model fitted in R
> using nlme package. In particular, what are the t-values that are given
> in an output, how are they calculated and based on what test? I guess it
> cannot be a simple Student t-test, otherwise how can the simple Student
> t-test test for significance of interactions, right? I cannot find this
> information in any of R help resources on linear-mixed models and I also
> checked few books.
  The t-statistics are the ratio of the previously quoted
Value (parameter estimate) and Std.Error columns;
the p value is a 2-sided test against
the null hypothesis that this t-statistic is drawn from
a standard t distribution with "DF" degrees of freedom, i.e.

2*pt(abs(tstat),df=DF,lower.tail=FALSE)

  You will find this referred to in the literature as a "Wald test".
For more information the best reference is Pinheiro and Bates 2000
(Springer)

Dear all,

 

I have a question about the output of linear mixed model fitted in R
using nlme package. In particular, what are the t-values that are given
in an output, how are they calculated and based on what test? I guess it
cannot be a simple Student t-test, otherwise how can the simple Student
t-test test for significance of interactions, right? I cannot find this
information in any of R help resources on linear-mixed models and I also
checked few books.

 

Example: the part of R output

 

model.b<-lme(diff~age+height,random=~1|field/replicate)

 

Linear mixed-effects model fit by REML

 

Fixed effects: diff ~ age + height_cm 

                Value Std.Error  DF   t-value p-value

(Intercept) 172.83559 27.094642 107  6.378958  0.0000

ageyoung     -5.28206 11.239981   4 -0.469935  0.6629

height_cm    -0.67662  0.450183 107 -1.502997  0.1358

 

Thank you very much for your help!

 

Best wishes,

Olja

 

 


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