[R-sig-ME] Wrong degrees of freedom in lmer model?

ONKELINX, Thierry Thierry.ONKELINX at inbo.be
Thu Apr 2 22:48:46 CEST 2009

Dear Ryan,

Targets needs 4 degrees of freedom because you omitted the intercept
from your model. A model with intercept needs only 3 degrees of freedom
because the intercept estimates the effect of the reference level.



ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek / Research Institute for Nature
and Forest
Cel biometrie, methodologie en kwaliteitszorg / Section biometrics,
methodology and quality assurance
Gaverstraat 4
9500 Geraardsbergen
tel. + 32 54/436 185
Thierry.Onkelinx at inbo.be 

To call in the statistician after the experiment is done may be no more
than asking him to perform a post-mortem examination: he may be able to
say what the experiment died of.
~ Sir Ronald Aylmer Fisher

The plural of anecdote is not data.
~ Roger Brinner

The combination of some data and an aching desire for an answer does not
ensure that a reasonable answer can be extracted from a given body of
~ John Tukey

-----Oorspronkelijk bericht-----
Van: r-sig-mixed-models-bounces at r-project.org
[mailto:r-sig-mixed-models-bounces at r-project.org] Namens Ryan Hope
Verzonden: donderdag 2 april 2009 21:52
Aan: r-sig-mixed-models at r-project.org
Onderwerp: [R-sig-ME] Wrong degrees of freedom in lmer model?

I am trying to use the lmer function to analyze the data from the
thesis experiment. The response variable is a continuous variable,
completion time. I have 2 fixed factors, number of targets (4-levels:
4,9,14,19) and displacement per second (DPS) (3-levels: 72.42, 84.99,
88.99). I also have 3 random variables, participants, block (which
group the replicates of a combination of fixed factors), and target
displacement. The R code that I am using is listed below:


Im not even sure if the lmer model I am using is 100% correct but it
seems the best to based on what I've read on
Anyway, I do not understand the degrees of freedom reported by the
anova function based on my model (shown below):

Analysis of Variance Table
        Df Sum Sq Mean Sq    F value
Targets  4  33393    8348 39411.3500
DPS      1      2       2     7.7816

Since there are 4 levels of the Target factor, shouldn't the Df for
Targets be 3 not 4?


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