[R-sig-eco] how to standardize one variable to control for another confounding variable

[李保平]

Dera Thierry,
     Thanks for the sugestion. Would you please give me more information about how to standardize one variable 
to control for the effect of another confounding one? it would be much helpful to expalin it with a example, as my statistic knowledge is limited. 

    I am looking for your reply. 

    with best wishes,

     Baoping


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>    1. help (lbp at njau.edu.cn)
>    2. Re: help (Thierry Onkelinx)
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> Message: 1
> Date: Thu, 19 May 2016 08:39:33 +0800
> From: "lbp at njau.edu.cn" <lbp at njau.edu.cn>
> To: r-sig-ecology <r-sig-ecology at r-project.org>
> Subject: [R-sig-eco] help
> Message-ID: <201605190839272047103 at njau.edu.cn>
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> Now I have difficulty to choose between two modelling approach. Here is th story. To examine the clutch size laid by a parasitic wasp into caterpillar hosts which varies in age and size, we exposed individual larvae of  three stages, after individually weighed, to parasitism, and the number of wasps emerged from each host (brood size) was recorded. Because host size and stage are mutually counfounding factors, the older the bigger, I have two modelling choices to separate the effect of stage and size. One is to make separate analyses, first analyzing the effct of stages in a model without the variable size, and then analyze the effect of the size under each stgae (several models); another approach is to make one analysis, put stage and size together in the same model to evaluate their separate and interacted effects on brood size.  For the later approach, the variable size actualy contains the inflormation about the stage, thus violating the independence assumation. I am won!
>  dering which appraoch is approriate.
>    
>    Thanks for any help. 
> 
>   Baoping
> 
> 
> 
> 
> E-mail: lbp at njau.edu.cn
> Phone: 86 (25) 84396394
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> ------------------------------
> 
> Message: 2
> Date: Thu, 19 May 2016 09:55:16 +0200
> From: Thierry Onkelinx <thierry.onkelinx at inbo.be>
> To: "lbp at njau.edu.cn" <lbp at njau.edu.cn>
> Cc: r-sig-ecology <r-sig-ecology at r-project.org>
> Subject: Re: [R-sig-eco] help
> Message-ID:
> 	<CAJuCY5wP7bdpDX6pPm2t6us1Zgsm0OnhDK0Z_Ji=EBr_BmX+Sg at mail.gmail.com>
> Content-Type: text/plain; charset="UTF-8"
> 
> Dear Baoping,
> 
> You could standardize the weight by stage. That would remove the
> confounding between weight and stage.
> 
> 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
> 
> 2016-05-19 2:39 GMT+02:00 lbp at njau.edu.cn <lbp at njau.edu.cn>:
> 
> > Now I have difficulty to choose between two modelling approach. Here is th
> > story. To examine the clutch size laid by a parasitic wasp into caterpillar
> > hosts which varies in age and size, we exposed individual larvae of  three
> > stages, after individually weighed, to parasitism, and the number of wasps
> > emerged from each host (brood size) was recorded. Because host size and
> > stage are mutually counfounding factors, the older the bigger, I have two
> > modelling choices to separate the effect of stage and size. One is to make
> > separate analyses, first analyzing the effct of stages in a model without
> > the variable size, and then analyze the effect of the size under each stgae
> > (several models); another approach is to make one analysis, put stage and
> > size together in the same model to evaluate their separate and interacted
> > effects on brood size.  For the later approach, the variable size actualy
> > contains the inflormation about the stage, thus violating the independence
> > assumation. I am won!
> >  dering which appraoch is approriate.
> >
> >    Thanks for any help.
> >
> >   Baoping
> >
> >
> >
> >
> > E-mail: lbp at njau.edu.cn
> > Phone: 86 (25) 84396394
> >
> >         [[alternative HTML version deleted]]
> >
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