[R-sig-ME] comparing 3 levels of fixed factor in lme4

[Obermeier Andrew]

Thank you John Maindonald.

For more than 2 levels of the experimental condition, I learned that usually we find an F to test mean differences across the levels of the condition.

In lme4, the model summary reports a t value, and I am replicating a study that uses lme4 to compare 3 levels of a fixed factor. My advising professor has told me that the t value can only be used to compare 2 means.

Andrew Obermeier




On Jul 30, 2012, at 8:00 PM, John Maindonald <john.maindonald at anu.edu.au> wrote:

> Perhaps Andrew has vaguely at the back of his mind the notion
> that for comparing >2 means, one should be using a multiple
> range test or an anova test, at all events if the aim is to achieve
> an experiment-wise 5% level.  Tests based on the individual
> t-statistics are not independent.  This is of course a somewhat
> controversial area.  
> John Maindonald             email: john.maindonald at anu.edu.au
> phone : +61 2 (6125)3473    fax  : +61 2(6125)5549
> Centre for Mathematics & Its Applications, Room 1194,
> John Dedman Mathematical Sciences Building (Building 27)
> Australian National University, Canberra ACT 0200.
> http://www.maths.anu.edu.au/~johnm
> 
> On 30/07/2012, at 8:38 PM, peter dalgaard wrote:
> 
>> 
>> On Jul 30, 2012, at 08:04 , Obermeier Andrew wrote:
>> 
>>> In lme4, in models with 3 levels of the fixed factor, each of these gets a t value comparing it to a reference level.
>>> 
>>> How is this done? 
>>> 
>>> It is my understanding that the t value can only be used to compare 2 means.
>>> 
>> 
>> Then your understanding is wrong, and you need to read a text on basic linear modelling theory. Nothing specifically mixed-model or even R relevant about that.
>> 
>> -- 
>> Peter Dalgaard, Professor,
>> Center for Statistics, Copenhagen Business School
>> Solbjerg Plads 3, 2000 Frederiksberg, Denmark
>> Phone: (+45)38153501
>> Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com
>> 
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>