[R] Using model operator in stepwise function's upper scope formula

[Yu Shao]

Hello:

I am doing forward stepwise analysis on the glm model. I am trying to use model 
operator in the "upper" scope formula, for example,          

                       scope=list(lower=~1,upper=~ .^2)

but the upper bound of the scope seems to be ignored and add1 is not performed 
at all, while if the terms are explicitly listed in the formula, the step 
function seems to work properly. Example:

> version 
platform sparc-sun-solaris2.9
arch     sparc               
os       solaris2.9          
system   sparc, solaris2.9   
status                       
major    1                   
minor    9.0                 
year     2004                
month    04                  
day      12                  
language R                   

> testdata
      effect  fa1 fa2    fa3
1  0.5054526 -1.4   1 -15.42
2  0.1366526 -2.5   3 -14.72
3  0.4798526 -1.1   3 -16.43
4  0.5566526 -4.6   2 -17.62
5  0.6568526 -3.5   7  -5.41
6  0.6653526  0.0   5 -11.88
7  0.6376526  0.0   5 -11.72
8  0.5203526 -2.9  -1 -14.72
9  0.4905526 -2.1   2 -17.68
10 0.6376526 -1.1   1 -13.39

> step (glm(effect ~ 1 , data=testdata), scope=list(lower=~1,upper=~.), 
direction='forward')
Start:  AIC= -5.89 
 effect ~ 1 


Call:  glm(formula = effect ~ 1, data = testdata) 

Coefficients:
(Intercept)  
     0.5287  

Degrees of Freedom: 9 Total (i.e. Null);  9 Residual
Null Deviance:      0.2178 
Residual Deviance: 0.2178       AIC: -5.89 

================================================================

But if I listed the term explicitly in the formula, then step function seemed to 
work correctly:

> step (glm(effect ~ 1 , data=testdata), 
scope=list(lower=~1,upper=~fa1+fa2+fa3), direction='forward')
Start:  AIC= -5.89 
 effect ~ 1 

       Df Deviance     AIC
+ fa3   1   0.1778 -5.9188
<none>      0.2178 -5.8901
+ fa2   1   0.2023 -4.6271
+ fa1   1   0.2065 -4.4194

Step:  AIC= -5.92 
 effect ~ fa3 

       Df Deviance     AIC
<none>      0.1778 -5.9188
+ fa1   1   0.1698 -4.3783
+ fa2   1   0.1770 -3.9605

Call:  glm(formula = effect ~ fa3, data = testdata) 

Coefficients:
(Intercept)          fa3  
    0.78310      0.01830  

Degrees of Freedom: 9 Total (i.e. Null);  8 Residual
Null Deviance:      0.2178 
Residual Deviance: 0.1778       AIC: -5.919 
================================================================

I've lookup the online manual of step function but couldn't find a solution. 
Thanks in advance,

Yu Shao
Wadsworth Center, New York State Department of Health,
Albany, NY 12208