R help - Jun 2003 - coefficient of logistic regression

Hello

in logistic regression,
I want to know that it is possible to get probability values of each 
predictors by
using following formula for each predictor one by one (keeping constant 
the others)
 <<< exp(coef)/(1+exp(coef)) >>>

thanks in advance

Ahmet Temiz


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At 11:54 AM 6/3/2003 +0300, orkun wrote:
>in logistic regression,
>I want to know that it is possible to get probability values of each 
>predictors by
>using following formula for each predictor one by one (keeping constant 
>the others)
><<< exp(coef)/(1+exp(coef)) >>>
Dear Ahmet,

This will almost surely give you nonsense, since it produces a fitted 
probability ignoring the constant in the model (assuming that there is 
one), setting other predictors to 0 and the predictor in question to 1. 
What is it that you want to do?

I hope that this helps,
  John

-----------------------------------------------------
John Fox
Department of Sociology
McMaster University
Hamilton, Ontario, Canada L8S 4M4
email: jfox at mcmaster.ca
phone: 905-525-9140x23604
web: www.socsci.mcmaster.ca/jfox

Ahmet  -

In a logistic regression model, fitted probabilities make
sense for individual cases (rows in the data set), as well
as for future cases (predictions) for which no outcome
(success or failure) has been observed yet.  Fitted
probabilities are calculated from the matrix formula:

  Pr[success]  =  exp( X %*% beta) / (1 + exp( X %*% beta)

where  X  is an [n x (p+1)] matrix, containing all p predictor
variables as columns, preceded by a column of 1s for the
intercept, and  beta  is the [(p+1) x 1] vector of logistic
regression coefficients.

One can interpret the sign and the magnitude of an individual
regression coeffient by saying that an increase of 1 unit in
predictor variable [i] will increase or decrease the odds of
success by a multiplier of  exp(beta[i]).  When  beta[i] > 0
the odds increase, because  exp(beta[i]) > 1,  and when
beta[i] < 0  the odds decrease, because  exp(beta[i]) < 1.

I hope this explanation helps.

-  tom blackwell  -  u michigan medical school  -  ann arbor  -

On Tue, 3 Jun 2003, orkun wrote:
> Hello
>
> in logistic regression,
> I want to know that it is possible to get probability values of each
> predictors by
> using following formula for each predictor one by one (keeping constant
> the others)
>  <<< exp(coef)/(1+exp(coef)) >>>
>
> thanks in advance
> Ahmet Temiz