Re: st: about residuals and coefficients

[Yuval Arbel <yuval.arbel@gmail.com>]

Ronan,

Thank you for the illuminating insights from Biology. It supports my
prior implication: It is desirable to reach equal conditions, but it
is practically impossible. All we can strive to achieve is to make the
experiment in the best way possible by controlling for (adjusting for)
as many variables as we can.

One good example for what is desirable and what is possible is
experiments made on twins (in Twinsboro - for example). Identical
twins are considered to be genetically identical. Yet recent
discoveries reveal they are not 100% genetically identical

On Sat, Sep 7, 2013 at 4:22 AM, Ronan Conroy <rconroy@rcsi.ie> wrote:
> On 2013 MFómh 7, at 01:42, David Hoaglin wrote:
>
>> Most experiments in social science cannot collect data that allows all
>> variables to be held constant.  A good design, however, may include
>> all combinations of two or more factors, so that one can study the
>> effect of one factor without changing the other factors.  Usually,
>> many additonal variables can only be observed.  Those are analyzed as
>> covariates (and adjusted for).
>
> I don't tend to believe in this. It implies that the same experimental units are studied under constant conditions, while in real life we study experimental units treated as identical (Wistar rats, for example) under conditions are are not really constant but in which random variation is tolerated (exact site of injection, nearness of rats' cage to radio playing some awful music all day…)
>
> Experiments in the prediction of lifespan of genetically identical roundworms observed in a controlled environment have consistently failed to predict lifespan based on any observable data such as nutrient intake, energy expenditure etc.
>
> See the brilliantly-argued paper by George Davey Smith -
> Davey Smith, G. Epidemiology, epigenetics and the “Gloomy Prospect”: embracing randomness in population health research and practice. Int J Epidemiol. 2011 Jun;40(3):537–62.
>
> Ronán Conroy
> rconroy@rcsi.ie
> Associate Professor
> Division of Population Health Sciences
> Royal College of Surgeons in Ireland
> Beaux Lane House
> Dublin 2
>
>
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-- 
Dr. Yuval Arbel
School of Business
Carmel Academic Center
4 Shaar Palmer Street,
Haifa 33031, Israel
e-mail1: yuval.arbel@carmel.ac.il
e-mail2: yuval.arbel@gmail.com
You can access my latest paper on SSRN at:  http://ssrn.com/abstract=2263398
You can access previous papers on SSRN at: http://ssrn.com/author=1313670

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