R help - Nov 2003 - what does this multinom error mean?

I have RedHat linux 9 with R 1.8.

I'm estimating models with multinom with a dependent variable that has 3 
different values.  Sometimes the models run fine and I can understand 
the results.

Sometimes when I put in another variable, I see an indication that the 
estimation did work, but then I can't get the summary method to work.  
It's like this:

 > votemn1 <-  multinom(vote~V023022+rep+V023027+ V023131,data=nes2002)
# weights:  18 (10 variable)
initial  value 914.045424
iter  10 value 474.831205
iter  20 value 449.612637
iter  20 value 449.612636
iter  20 value 449.612636
final  value 449.612636
converged

 > summary(votemn1)
Error in model.frame(formula, rownames, variables, varnames, extras, 
extranames,  :
    invalid variable type

In this model, rep is a dichotomous (0,1) variable indicating if a 
person is a republican or not. If I drop that variable, the model does 
run and the summary method produces estimates & standard errors.

 > votemn2 <-  multinom(vote~V023022+V023027+ V023131,data=nes2002)
# weights:  15 (8 variable)
initial  value 917.341261
iter  10 value 529.137064
final  value 527.178682
converged

 > summary(votemn2)
Call:
multinom(formula = vote ~ V023022 + V023027 + V023131, data = nes2002)

Coefficients:
  (Intercept)    V023022    V023027     V023131
3  -2.2033403  0.9227144 -0.3835378 0.017960208
5  -0.8411559 -0.1853416 -0.2174085 0.005808468

Std. Errors:
  (Intercept)    V023022   V023027    V023131
3   0.5883961 0.07054401 0.0894393 0.05655965
5   1.2438595 0.14582161 0.1963035 0.12453307

Residual Deviance: 1054.357
AIC: 1070.357
...

-- 
Paul E. Johnson                       email: pauljohn at ukans.edu
Dept. of Political Science            http://lark.cc.ukans.edu/~pauljohn
University of Kansas                  Office: (785) 864-9086
Lawrence, Kansas 66045                FAX: (785) 864-5700

Note that is a *model.frame* error, not a multinom one.

Don't use the names of R functions as variable names!  I am sure that is
in the introductory documentation, and it is certainly stressed in my
books.  Although I cannot be sure it seems very likely (perhaps because of
the scoping induced by namespaces) that your `rep' is being matched to the
function.

On Thu, 13 Nov 2003, Paul E. Johnson wrote:
> I have RedHat linux 9 with R 1.8.
> 
> I'm estimating models with multinom with a dependent variable that has
3
> different values.  Sometimes the models run fine and I can understand 
> the results.
> 
> Sometimes when I put in another variable, I see an indication that the 
> estimation did work, but then I can't get the summary method to work.  
> It's like this:
> 
>  > votemn1 <-  multinom(vote~V023022+rep+V023027+
V023131,data=nes2002)
> # weights:  18 (10 variable)
> initial  value 914.045424
> iter  10 value 474.831205
> iter  20 value 449.612637
> iter  20 value 449.612636
> iter  20 value 449.612636
> final  value 449.612636
> converged
> 
>  > summary(votemn1)
> Error in model.frame(formula, rownames, variables, varnames, extras, 
> extranames,  :
>     invalid variable type
> 
> In this model, rep is a dichotomous (0,1) variable indicating if a 
> person is a republican or not. If I drop that variable, the model does 
> run and the summary method produces estimates & standard errors.
> 
>  > votemn2 <-  multinom(vote~V023022+V023027+ V023131,data=nes2002)
> # weights:  15 (8 variable)
> initial  value 917.341261
> iter  10 value 529.137064
> final  value 527.178682
> converged
> 
>  > summary(votemn2)
> Call:
> multinom(formula = vote ~ V023022 + V023027 + V023131, data = nes2002)
> 
> Coefficients:
>   (Intercept)    V023022    V023027     V023131
> 3  -2.2033403  0.9227144 -0.3835378 0.017960208
> 5  -0.8411559 -0.1853416 -0.2174085 0.005808468
> 
> Std. Errors:
>   (Intercept)    V023022   V023027    V023131
> 3   0.5883961 0.07054401 0.0894393 0.05655965
> 5   1.2438595 0.14582161 0.1963035 0.12453307
> 
> Residual Deviance: 1054.357
> AIC: 1070.357
> ...
> 
> 
-- 
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