[Rd] Extractor function for standard deviation.

[Roger D. Peng]

I'm not sure the idea of having a generic 'sd' extract the residual standard 
deviation from a linear model quite jives with me.  Personally, I think a 
generic function with name like 'residSD' or something similar would be better.

Either way, I think it would be nice to have some function like this.

-roger

Rolf Turner wrote:
> I have from time to time seen inquiries on r-help in respect of
> how to obtain the estimated standard deviation from the output of
> fitting a linear model.  And have had occasion to want to do this
> myself.
> 
> The way I currently do it is something like summary(fit)$sigma
> (where fit is returned by lm()).
> 
> It strikes me that it might be a good idea to have an extractor
> function, analogous with coef() and resid(), to dig out this value.
> This would be in keeping with the R philosophy of ``don't muck
> about with the internal components of objects returned by functions,
> because the internal structure of such objects might change in future
> releases''.
> 
> I'm not sure what the name of the extractor function should be.
> One idea would be to make sd() generic, and create a function
> sd.lm() which could currently have code
> 
> 	sd.lm <- function(x,...) {
> 		summary(x)$sigma
> 	}
> 
> The sd.default() function would have the code of the current sd().
> 
> Does this idea have any merit?
> 
> 		cheers,
> 
> 			Rolf Turner
> 
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-- 
Roger D. Peng  |  http://www.biostat.jhsph.edu/~rpeng/