[R-sig-ME] Linear mixed model - heterogeneity

[Etn bot]

@ Paul, many thanks for your response, I will check out the link you sent

On 30 October 2015 at 13:56, Paul Buerkner <paul.buerkner at gmail.com> wrote:

> You may also try the brms package, which has a hurdle_gamma family that
> might be helpful to you.
>
> A sample hurdle_gamma model (using the epilepsy data of brms) may look
> like this:
>
> fit <- brm(count ~ 0 + trait * (log_Age_c + log_Base4_c * Trt_c)
>             + (0+trait||patient),
>             data = epilepsy, family = hurdle_gamma("log"))
>
> The reserved variable "trait" has to levels, one for the gamma part and
> one for the bernoulli part modeling zeros.
>
> Currently, hurdle_gamma models are only available in the github version of
> brms to be installed via
>
> library(devtools)
> install_github("paul-buerkner/brms")
>
> Since brms is based on Stan, you will need a C++ compiler. Instructions on
> how to get one are presented at the end of the README on
> https://github.com/paul-buerkner/brms.
>
>
>
> 2015-10-23 16:15 GMT+02:00 Etn bot <etnbot1 at gmail.com>:
>
>> I have a run a linear mixed effects model in R to model clinical data,
>> however this model is heteroscedastic (as there excess zeros in the
>> response variable)....
>>
>> I have tried transforming the data (log transform) and (sqrt), however
>> neither transformation resolve the issue (see residual versus fitted value
>> plot). I have not used cox proportional hazards model as the data is not
>> time-to-event data, the data measures force and there are a large number
>> of
>> observations have a reading of zero. I cannot exclude these readings as
>> they are valid.
>>
>> I have found a R package that runs Tobit regression (AER), however this
>> will not accommodate the random effects in the model. I cannot find any R
>> packages that run Weibull mixed effects models (or gamma mixed effects
>> models)...
>>
>> Does anyone know if there is a package to run these type of models? (or
>> can
>> they suggest any alternative approach).
>>
>> Many thanks
>>
>>
>> Etn
>>
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>>
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>>
>
>

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