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testing random slopes in three-level models & error message

2 messages · Charlotte Arndt, Thierry Onkelinx

Original
Charlotte Arndt
# 
Hi all,

I have two questions, one regarding the testing of random slopes in 
three-level models and one regarding an error message:

My data have a three-level structure: Items (level 1, n=6111) are nested 
within measurement occasions (level 2, n=2111), and measurement 
occasions are nested within persons (level 3, n=84). I assume a 
curvilinear relationship between one predictor and one outcome (both at 
level 1), so I included a linear and a squared term as predictors in my 
model. I am mainly interested in the fixed effects but to find out the 
"best" model to report, I want to test whether random slopes are needed 
at level 2 and/or level 3.
I wonder whether there is any "best practice" in which order the random 
slopes should be tested in three-level models?

I tried to compute a full model (random slopes for all terms at both 
levels) to compare this with models in which only one of the four random 
terms was fixed (this was done for all four possible random slopes).

Using lme4, I got an error message with regard to the full model :
PRED.linear + PRED.squared| ID.L2) + (1 +

PRED.linear + PRED.squared  | ID.L3), data)
Error: number of observations (=6111) <= number of random effects 
(=6333) for term (1 + PRED.linear + PRED.squared | ID.L2); the 
random-effects parameters and the residual variance (or scale parameter) 
are probably unidentifiable


If more information is needed, please let me know.
Thanks,

Charlotte
Thierry Onkelinx
# 
Dear Charlotte,

The error message seems to be quite clear: you're model is too complex
for your dataset. So either simplify to model or get more data.

Best regards,
ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek / Research Institute for Nature
and Forest
team Biometrie & Kwaliteitszorg / team Biometrics & Quality Assurance
Kliniekstraat 25
1070 Anderlecht
Belgium

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 data. ~ John Tukey


2015-09-15 14:47 GMT+02:00 Charlotte Arndt <arndtch at uni-landau.de>: