[R-sig-ME] strange model fit- help

[marKo]

Hi.

I have fitted a relatively complicated model to electrodermal data (a 
simple resting and stimulus situation). The data summary follows.

 > summary(data)
        id             sc              t              stim
  g1_1   :  49   Min.   :26798   Min.   :  1.0   before  :3201
  g1_12  :  49   1st Qu.:32299   1st Qu.:123.0   after   :1543
  g1_13  :  49   Median :32486   Median :245.0
  g1_14  :  49   Mean   :32253   Mean   :244.9
  g1_15  :  49   3rd Qu.:32587   3rd Qu.:367.0
  g1_2   :  49   Max.   :32761   Max.   :489.0
  (Other):4450

id (person), and stim are factors, t is time (in s) and sc is skin 
conductance level. Sc distribution is quite negatively asymmetrical at 
the dataset level, although not that bad at the id level. As the 
stimulus occur at a specified time, those two variables are correlated 
(0.81).

The model follows.

m1<-lmer(sc~1+t+I(t^2)+stim+stim:t+stim:I(t^2)+(1+t+I(t^2)+stim+stim:t+stim:I(t^2)|id), 
data=data)

Here goes the summary.

 > summary(m1)
Linear mixed model fit by maximum likelihood  ['lmerMod']
Formula: sc ~ 1 + t + I(t^2) + stim + stim:t + stim:I(t^2) + (1 + t + 
I(t^2) + stim + stim:t + stim:I(t^2) | id)
    Data: data

      AIC      BIC   logLik deviance df.resid
  62325.9  62506.9 -31134.9  62269.9     4716

Scaled residuals:
      Min       1Q   Median       3Q      Max
-24.3783  -0.1551  -0.0074   0.1392  12.5288

Random effects:
  Groups   Name          Variance  Std.Dev.  Corr
  id       (Intercept)   4.681e+04 2.164e+02
           t             7.925e+00 2.815e+00  1.00
           I(t^2)        2.559e-05 5.059e-03 -0.87 -0.87
           stim.L        1.591e+05 3.989e+02 -0.12 -0.12  0.17
           t:stim.L      1.105e+00 1.051e+00 -0.58 -0.58  0.78  0.33
           I(t^2):stim.L 2.367e-05 4.865e-03  0.06  0.06 -0.21 -0.76 -0.45
  Residual               2.049e+04 1.432e+02
Number of obs: 4744, groups:  id, 97

Fixed effects:
                 Estimate Std. Error t value
(Intercept)    2.960e+04  1.637e+02  180.85
t              1.291e+01  8.493e-01   15.20
I(t^2)        -1.579e-02  1.110e-03  -14.21
stim.L        -3.956e+03  2.329e+02  -16.98
t:stim.L       2.047e+01  1.136e+00   18.01
I(t^2):stim.L -2.477e-02  1.478e-03  -16.76

Correlation of Fixed Effects:
             (Intr) t      I(t^2) stim.L t:st.L
t           -0.886
I(t^2)       0.811 -0.966
stim.L       0.972 -0.930  0.868
t:stim.L    -0.989  0.911 -0.826 -0.973
I(t^2):st.L  0.917 -0.858  0.761  0.870 -0.947

The fit of the model is quite good (pseudo r2 is 0.96), but have some 
problems:
1: quite “extreme” residuals (-24.3783,  12.5288)
2: quite high correlations among random effects
3: lousy qqplot (apart from the perfect fit on the  from -2 to +2 std 
normal quantiles)

Help please? What is wrong with the model (something is, I’m sure).	




-- 
Marko Tončić, PhD
Postdoctoral research assistant
University of Rijeka
Faculty of Humanities and Social Sciences
Department of Psychology
Sveucilisna avenija 4, 51000 Rijeka, CROATIA
e-mail: mtoncic using ffri.hr