[R-sig-ME] Variance components questions

[ONKELINX, Thierry]

Dear Karim,

I assume that the samples are reference samples with known concentration? In that case I would create a new variable: logRatio = log(measurement/known) and model that as logRatio ~ operator + (1|day/run) + (1|sample). This new variable is the relative bias of the measurements. 

Best regards,

Thierry

----------------------------------------------------------------------------
ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek
team Biometrie & Kwaliteitszorg
Gaverstraat 4
9500 Geraardsbergen
Belgium

Research Institute for Nature and Forest
team Biometrics & Quality Assurance
Gaverstraat 4
9500 Geraardsbergen
Belgium

tel. + 32 54/436 185
Thierry.Onkelinx at inbo.be
www.inbo.be

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
  

> -----Oorspronkelijk bericht-----
> Van: r-sig-mixed-models-bounces at r-project.org 
> [mailto:r-sig-mixed-models-bounces at r-project.org] Namens Karim Lounes
> Verzonden: dinsdag 22 februari 2011 0:01
> Aan: r-sig-mixed-models at r-project.org
> Onderwerp: [R-sig-ME] Variance components questions
> 
> Hi All,
> I am a biochemist with limited background in statistics, so 
> please forgive the naivety of my question.
> I am interested in estimating the precision of a measurement 
> system and the variance components of three factors.
> This is how the experiment was designed: 5 samples, each 
> considered a level were measured. Each sample was measured 
> twice a day (run) for 10 days (days) by two different 
> operators (operator). Each measurement was replicated 3 
> times. The factor (days) is not continuous as the goal was to 
> generalize the estimation of the precision of the measurement 
> system for all days.
> I have read a number of posts on this list and also read Mr. 
> bates online book of lem4.
> >From what I understood the model would be a crossed design 
> and I  run 
> >the
> following analysis:
> 
> Measurement ~ samples + (1|days) +  (1|run) +(1|operator)
> 
> Is this model appropriate? and is there a way to gauge the 
> precision of the the estimates of the variance components by 
> a confidence interval?
> 
> Thank you for your help
> 
> Regards
> 
> Karim
> 
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