Re: [R] Reconstructing LD function

From: Prof Brian Ripley <>
Date: Fri 01 Jul 2005 - 23:44:40 EST

On Fri, 1 Jul 2005 wrote:

> On Mon, 2005-06-27 at 13:18, Prof Brian Ripley wrote:
>> On Mon, 27 Jun 2005 wrote:
>>> in an LDA analysis with n groups n-1 LD functions result. Implicitly this
>>> defines an LD fucntion for the last group. Does there exist code already
>>> to explictly construct this LD function?
> Thank you for the quick reply.
>> What `LDA analysis' are our discussing here? (LDA is usually
>> `linear discriminant analysis', so what did you mean and what R function
>> are you nor referring to?)
>> R has lda in package MASS, and that works with n LD functions. To reduce
>> it to n-1, subtract the last one from the others, in which case LD_n == 0.
> Indeed I have been using the MASS::lda package.
>> Anything you do in LD analysis only depends on differences in LD
>> functions, and there really are n of them. With two groups one is
>> conventionally taken to be zero (the first, usually, not the last).

> How is the classifcation decision reached from the LD functions? Are
> those what is known as "linear Fisherian discriminant functions"? If so,
> I'm not positive about why one of these functions can be set to 0.

What I did say:

`Anything you do in LD analysis only depends on differences in LD functions'

So subtracting any one function from the others does not change the differences.

LD is not about classification, and Fisher did not do classification, nor did he use more than 2 classes. I suspect your difficulty is going to be clearing your preconceptions. lda() is support software for a book which does explain the relationship between Fisher's discrimination and classification: please consult it for the background.

Brian D. Ripley,        
Professor of Applied Statistics,
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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Received on Fri Jul 01 23:49:39 2005

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