[R-sig-ME] How to fix the value of random intercepts (lmer/MCMCglmm)

[Ben Bolker]

  Fitting a cumulative distribution is sometimes statistically dicey,
since the successive values will be correlated; is there a reason you
can't take first differences?

  Technically speaking, to do this in lme4 I think you would want to use
a slope-only model and include an intercept for individuals as an offset:

lmer (Y ~Intercept + Continuous Variable +
    offset(indiv_intercept)+
   (Continuous Variable +0|Individual)




On 14-08-25 11:33 AM, Jarrod Hadfield wrote:
> Hi Sam,
> 
> You could do this in MCMCglmm but it sounds like it might (possibly) be
> a bad idea. Could you give more details on how Y is actually obtained?
> 
> Cheers,
> 
> Jarrod
> 
> 
> Quoting "PATRICK, Samantha" <spatrick at glos.ac.uk> on Mon, 25 Aug 2014
> 15:16:27 +0000:
> 
>> Hi All
>>
>> I am fitting a basic linear regression, where I want to estimate a
>> single population intercept and slope.  In addition I am fitting
>> random intercepts and slopes such that:
>>
>> lmer (Y ~Intercept + Continuous Variable + (Continuous Variable
>> |Indiviudal Group))
>>
>> However the exact value of the individual group intercepts is known
>> from the data set.  The reasons for this are a little involved but
>> essentially Y is a cumulative total and so at the intercept I want to
>> fit the actual cumulative total at this point for each individual.  It
>> is important as the slope per individual needs to be constrained to
>> pass through the actual intercept per individual.
>>
>> So I want to fit this model, estimating the population intercept and
>> slope.  I then want to fix the individual group deviation from the
>> population intercept (random intercepts), and from this model extract
>> estimates of individual group random slopes.
>>
>> I have been unable to find any examples of fixing intercepts, unless
>> they are fixed as a constant.  Is it possible to code the model in
>> such a way? The model can be run in MCMCglmm or lmer  which ever
>> package would allow me to constrain the intercepts.
>>
>> Thanks
>>
>> Sam​
>>
>>
>> Dr Samantha Patrick
>> Research Fellow
>> Biosciences QU116
>> Francis Close Hall Campus
>> University of Gloucestershire
>> Cheltenham, GL50 4AZ, UK
>>
>> Research Associate: OxNav, University of Oxford
>>