[R-sig-ME] question about an unbalanced design using lmer
Henrik Singmann
singmann at psychologie.uzh.ch
Mon Oct 17 14:28:32 CEST 2016
PS: Sorry for sending this out three times. My mail client has some
problems with gmane lately...
Am 17.10.2016 um 12:17 schrieb Henrik Singmann:
> Dear Xiyue and Thiery,
>
> While more data points may affect the estimation process in that way
> they do not seem to affect the fixed-effects estimates in that way. To
> be more precise, the fixed effect estimate seems to correspond to the
> unweighted mean (i.e., the mean in which each level of the random effect
> is weighted equally) and not to the weighted mean (in which each data
> point is weighted equally).
>
> I had a similar problem some time ago:
> https://stat.ethz.ch/pipermail/r-sig-mixed-models/2014q3/022478.html
>
> Thanks to the help of Jake Westfall I was able to get the desired result
> (i.e., a fixed-effect estimate corresponding to the weighted mean), by
> adding group size as fixed effect to my model, see:
> https://stat.ethz.ch/pipermail/r-sig-mixed-models/2014q3/022481.html
>
> There might be other approaches to achieve this as well (i.e., some
> post-fit weighting), but I am not sure how to implement this (perhaps
> using lsmeans somehow).
>
> I hope this helps,
> Henrik
>
>
> Am 17.10.2016 um 11:09 schrieb Thierry Onkelinx:
>> Dear Xiyue,
>>
>> Don't think in terms of cells but in terms of observations. The model
>> tries
>> to minimise the residuals. So combinations with more observations have
>> more
>> residuals and thus a stronger impact on the MSE.
>>
>> 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
>>
>> 2016-10-12 19:48 GMT+02:00 Xiyue Liao
>> <liaoxiyue2011 at gmail.com>:
>>
>>> Hi,
>>>
>>> I'm using lmer in the R package lme4 to do a one-way anova analysis
>>> with a
>>> fixed effect term and a random effect term. So the fixed effect is about
>>> four medical conditions and the random effect is about randomly sampled
>>> donors. Now for some combinations of donors and medical conditions,
>>> there
>>> are more than one measurement, which makes the whole design
>>> unbalanced. I
>>> think that lmer can handle such a case, and I have run the code
>>> without any
>>> error message. However, I don't understand how this routine put
>>> weight on
>>> the cells with more measurements than other cells. Could you give me
>>> some
>>> hint?
>>>
>>> Thanks in advance for your help.
>>>
>>> Sincerely,
>>>
>>> Xiyue
>>>
>>> [[alternative HTML version deleted]]
>>>
>>> _______________________________________________
>>> R-sig-mixed-models at r-project.org mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-sig-mixed-models
>>>
>>
>> [[alternative HTML version deleted]]
>>
>
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