Dear all

I have ordinal data (from a questionnaire with 4 levels) for 2 groups (30 subjects each) and 2 timepoints. So I used a cumulative link mixed model to fit the data (nr = subject number). 

mod_FV<-clmm2(FV~GruppeVerbelendung+Timepoint,random=nr,data=data,Hess=TRUE,nAGQ=10,na.action=na.omit)    
mod_FV1<-clmm2(FV~GruppeVerbelendung,random=nr,data=data,Hess=TRUE,nAGQ=10,na.action=na.omit)    


For some questions (e.g. FV) I had the following output with NANs instead of p-values

Call:
clmm2(location = FV ~ GruppeVerbelendung * Timepoint, random = nr, 
    data = data, na.action = na.omit, Hess = TRUE, nAGQ = 10)

Random effects:
            Var      Std.Dev
nr 5.881958e-07 0.0007669392

Location coefficients:
                               Estimate Std. Error z value Pr(>|z|)
GruppeVerbelendung1            -0.0178      NaN        NaN NA      
Timepoint1                     18.6316      NaN        NaN NA      
GruppeVerbelendung1:Timepoint1  0.3505      NaN        NaN NA      

No scale coefficients

Threshold coefficients:
    Estimate Std. Error z value
1|2 20.7741      NaN        NaN
2|3 20.9486      NaN        NaN

log-likelihood: -24.01783 
AIC: 60.03566 
Condition number of Hessian: 5479162.40 
(4 observations deleted due to missingness)
Warnmeldung:
In sqrt(diag(vc)) : NaNs wurden erzeugt

However when comparing the models (mod_FV, mod_FV1) I obtain a usable result (p-val) even though the models seem not to be a good fit (high AIC and condition number of Hessian).

 anova(mod_FV,mod_FV1)
Likelihood ratio tests of cumulative link models

Response: FV
                                 Model Resid. df -2logLik   Test    Df LR stat.      Pr(Chi)
1             GruppeVerbelendung |  |        112 58.44384                                   
2 GruppeVerbelendung + Timepoint |  |        111 27.78259 1 vs 2     1 30.66125 3.072406e-08

What could be the problem? Timepoint 1 was always scored with level 1 (60 of 60 subjects had  scored 1) and during timepoint 2 subjects scored level 1-3. Might it be a problem that I only have 1 type of scoring level for timepoint 1?
Can I use the results obtained by Likelihod ratio test for clm even if the model was not a good fit? What could I do instead?


Thank you so much for your help and all the best
Caroline




 		 	   		  
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