[R-sig-ME] Changing reference level in clmm() ordinal regression

Thierry Onkelinx thierry.onkelinx at inbo.be
Fri Sep 22 14:38:46 CEST 2017

Dear Jessie,

All models have an identical fit. They only differ in the
parametrisation. "a3" estimates the difference between "a3" and the
reference. Hence, changing the reference results in a different
interpretation of the parameter and thus a different estimate.

Best regards,

ir. Thierry Onkelinx
Statisticus/ Statiscian

Vlaamse Overheid / Government of Flanders
Team Biometrie & Kwaliteitszorg / Team Biometrics & Quality Assurance
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2017-09-22 11:41 GMT+02:00 Jessie Barker <jessiebarker op gmail.com>:
> I have a couple of questions that I asked on StackExchange and someone
> suggested that I ask this mailing list. The post on StackExchange is here (
> https://stats.stackexchange.com/questions/304092/changing-reference-level-in-clmm-ordinal-regression-in-r),
> but I am summarizing it below:
> I’m analyzing data from a questionnaire where participants had to rank
> three answers to each question (e.g. 1 = most likely, 3 = least likely).
> They had to give a different rank to each answer, so each question has one
> answer ranked 1, one ranked 2, and one ranked 3. The set of three answers
> is the same for each questions, and there were seven questions.
> I want to know whether participants give different ranks to different
> answers, and whether that is affected by question. Here's my model:
> model1 <- clmm(rank ~ answer + answer:question + (1+answer|participant),
> data = mydata)
> (I don’t have question as a fixed effect, because for each question
> participants had to give a 1, 2 and 3 rank, so question alone does not
> affect rank.)
> My first question is whether I’ve set up the model correctly, as I don’t
> have any experience with ordinal regression. When I look at coef(model1),
> all participants seem to have the same intercepts and coefficients for the
> different answers, which is not what I thought should happen (I thought I
> was setting up a model with random intercepts and random slopes).
> My second question is about changing the reference level of question and
> answer. When I look at summary(model1), it uses answer a1 and question q1
> as the reference levels, so I ran the model again using different answers
> and questions as reference levels.
> When I run the model again using a different answer as the reference level,
> the coefficients for the fixed effects are the same, but the random effects
> and threshold coefficients are quite different.
> When I run the model using a different question as the reference level, the
> coefficients for the fixed effects are quite different, but the random
> effects are exactly the same, and the threshold coefficients relatively
> similar.
> Could someone please help me understand what’s going on here?
> Thanks in advance,
> Jessie Barker
>         [[alternative HTML version deleted]]
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