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GEOSTATS: Discriminant Analysis

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• If I want to discriminate between two groups ( A and B). Each group has two variable X and Y ( those variables has different unit ).Between A and B has
Message 1 of 4 , Jun 19, 1998
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If I want to discriminate between two groups ( A and B). Each group has
two variable X and Y ( those variables has different unit ).Between A
and B has different mean value of X and Y. If I standardize the
variables, each group will contain zero mean of variable X and Y. So
the difference matrix A and B is Zero.
"Do we have to standardize the variables before applying
discriminant analysis ?"

=palapa=

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• Dear palapa: No, you don t. A discriminant function is obtained as a linear combination of the variables like S=aX+bY+c and you are supposed to choose a
Message 2 of 4 , Jun 21, 1998
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Dear palapa:

No, you don't. A discriminant function is obtained
as a linear combination of the variables like S=aX+bY+c
and you are supposed to choose a critical value, say G, to
discriminate between the groups depending on whether
S>G or S<G.
Using standardized variables will change only the constant c.

I hope this brief note will help you.

Sincerely yours

Tsuyoshi

> If I want to discriminate between two groups ( A and B). Each group has
> two variable X and Y ( those variables has different unit ).Between A
> and B has different mean value of X and Y. If I standardize the
> variables, each group will contain zero mean of variable X and Y. So
> the difference matrix A and B is Zero.
> "Do we have to standardize the variables before applying
> discriminant analysis ?"
>
> ==
>
>
>
>
> --
> *To post a message to the list, send it to ai-geostats@....
> *As a general service to list users, please remember to post a summary
> of any useful responses to your questions.
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---------------------------------------------------------------

Tsuyoshi Nakamura
E-mail: nakamura-t@...-u.ac.jp
Fax: 81-95-8431782, Phone: 81-95-8436374
Professor of Biostatistics and Mathematics
Faculty of Environmental Studies
Nagasaki University,　Nagasaki 852, JAPAN

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• ... As far as I know (AFAIK): Malahanobis distance, used in the classification/discrimination algorithm, take care of standardization. Moreover, why
Message 3 of 4 , Jun 21, 1998
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On Sat, 20 Jun 1998, Palapa wrote:

> If I want to discriminate between two groups ( A and B). Each group has
> two variable X and Y ( those variables has different unit ).Between A
> and B has different mean value of X and Y. If I standardize the
> variables, each group will contain zero mean of variable X and Y. So
> the difference matrix A and B is Zero.
> "Do we have to standardize the variables before applying
> discriminant analysis ?"
>
> =palapa=
>
As far as I know (AFAIK): Malahanobis distance, used in the
classification/discrimination algorithm, take care of standardization.
Moreover, why standardize by group ? If you do such a thing, you can only
guess that the two correlation matrix will differ, else you'll get not
any difference between the two groups, but discriminant analysis won't
help in such case.

Recall that the assumption here is that the two covariance/correlation
matrixes (spelled right ?) are equal, else you should use quadratic
discriminant analysis or, to get better results, regularized discriminant
analysis.

To summarize: don't standardize.

A quite good and free software (MAC and Win95/NT) for Linear Discriminat
Analysis and much more (no cross-validation):

point to

http://pbil.univ-lyon1.fr/

and go to

hope that help

federico spinazzi
federico@...

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• Hi, I ve run a discriminant analysis using SPSSx and have calculated the discriminant scores. How do I calculate the probability that a case with a
Message 4 of 4 , Oct 20 4:25 PM
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Hi,

I've run a discriminant analysis using SPSSx and have calculated the
discriminant scores. How do I calculate the probability that a case with a
discriminant score of D belongs to a particular group. SPSSx will do the
calculation for me but I would prefer to be able to do it myself.

An old SPSSx Advanced Statistics Guide talks about using Bayes' Rule and
asks the reader to refer to Tatsuoka (1971). The applications Guide for
SPSS version 8.0 suggests computing the Mahalanobis distance. Either way
the calculations are not completed in the text leaving me to find my own
way.

Regards