[R-sig-ME] Linear mixed model - heterogeneity (Etn bot)

[Etn bot]

@ Alain, many thanks for your response.... I will check out this type of
model

On 30 October 2015 at 07:35, Highland Statistics Ltd <highstat at highstat.com>
wrote:

>
>
>
>
> ----------------------------------------------------------------------
>>
>> Message: 1
>> Date: Fri, 23 Oct 2015 15:15:45 +0100
>> From: Etn bot <etnbot1 at gmail.com>
>> To: r-sig-mixed-models at r-project.org
>> Subject: [R-sig-ME] Linear mixed model - heterogeneity
>> Message-ID:
>>         <
>> CAF79uvkRGaWXkzjPz9grTRhdQSVcqUmLrB+5QWUNS76JJLwmYg at mail.gmail.com>
>> Content-Type: text/plain; charset="UTF-8"
>>
>> 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
>>
>
> Hi,
>
> Run a hurdle model that consists of:
>
> 1. A Logistic regression model on the absence/presence data (e.g. using
> glmer).
> 2. A Gamma GLMM on the presence only data
>
> Then figure out the mean and variance of the zero altered Gamma
> distribution so that you have the fitted values of the combined model.
>
> Alain
>
> PS. This is also part of an exercise in the following course (which will
> run next week in Spain)..;-)
>
> http://highstat.com/Courses/Flyers/Flyer2015_11Elche.pdf
>
>
>
>
>
> --
> Dr. Alain F. Zuur
>
> First author of:
> 1. Beginner's Guide to GAMM with R (2014).
> 2. Beginner's Guide to GLM and GLMM with R (2013).
> 3. Beginner's Guide to GAM with R (2012).
> 4. Zero Inflated Models and GLMM with R (2012).
> 5. A Beginner's Guide to R (2009).
> 6. Mixed effects models and extensions in ecology with R (2009).
> 7. Analysing Ecological Data (2007).
>
> Highland Statistics Ltd.
> 9 St Clair Wynd
> UK - AB41 6DZ Newburgh
> Tel:   0044 1358 788177
> Email: highstat at highstat.com
> URL:   www.highstat.com
>
>

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