[R] LDA cutoff value

Prof Brian Ripley ripley at stats.ox.ac.uk
Fri Sep 23 19:05:07 CEST 2011

On Fri, 23 Sep 2011, B Jessop wrote:


> Hello, I have run a linear discriminant analysis for the simple 2 
> group case using the MASS package lda() function.  With priors fixed 
> at 0.5 and unequal n for each group, the output basically provides 
> the group means and the LD1 value.  There is no automatic output of 
> the cutoff (decision boundary) value used to classify values of the 
> response variable into the different groups.  I have tried various 
> unsuccessful approaches to extract this value.  It is obvious that 
> in the simple 2 group case the value will be close to the mean of 
> the 2 group means and that the LD1 value is involved (perhaps grand 
> mean * LD1?).  I am probably missing (misunderstanding?) the obvious 
> and would appreciate being educated in this matter.  Thanks. 
> Regards,BJ

Yes: 'discrimination' is not 'classification'.  However, this and the 
way to use LDA for classification are covered in the book for which 
MASS is support software, so please do your homework from that book.

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Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
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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