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Studentized residuals not always available with mixed model?

anne_sa

Community Trekker

Joined:

Feb 24, 2016

Hello,

 

I have a theroretical question here.

Sometimes when I run a mixed model from the fit model platform and then save the residuals ans studentized residuals, a large number of rows have a value of residuals but missing value for studentized residuals and I am wondering why...

 

After some researches, I have found that studentized residuals can be obtainded from the residuals with this formula:

 

stud res [i] = res[i] / (RMSE * √(1-h[i]))

where h[i] is the i th diagonal element of the hat matrix (which can also be saved in the data table)

 

From what I have read the diagonal elements of the hat matrix are supposed to be less than 1.

However sometimes I observe that there are greater. Thus it is not possible to calculate the square root of negative numbers and we get missing values for studentized residuals.

 

Is it a particular case? Is it because I have random effects in my model? Is the calculation of the hat matrix different in that case? Should we rather focus on the residuals?

 

I am a bit lost about that...

Any help would be appreciate!

 

Thanks in advance