help with step()

Folks:
 
I'm having trouble doing a forward variable selection using step()
 
First, I fit an initial model:
 
fit0  <-  glm ( est~1 , data=all, subset=c(n>=25) )
 
then I invoke step():
 
fit1 <- step( fit0 , scope=list(upper=est~ pcped + pchosp + pfarm
,lower=est~1))
 
 
I get the error message:  Error in eval(expr, envir, enclos) : invalid
'envir' argument
 
I looked at the documention on step(). There is no 'envir' arguement.
 
Can anyone shed light on what I'm doing wrong? 
 
Thanks,
Phil Smith
CDC

 

Could be your "data" argument. ?all is a base function. Try re-naming to 
something else. Here's an example:

set.seed(1)
z <- all <- data.frame(x = rnorm(10), y = rnorm(10))

glm.all <- glm(y ~ 1, data = all)
step(glm.all, list(upper = y ~ x, lower = y ~ 1))

# Start:  AIC= 32.67
# y ~ 1
#
# Error in eval(expr, envir, enclos) : invalid 'envir' argument

glm.z <- glm(y ~ 1, data = z)
step(glm.z, list(upper = y ~ x, lower = y ~ 1))

# Start:  AIC= 32.67
#  y ~ 1
#
#        Df Deviance    AIC
# <none>      10.295 32.669
# + x     1    8.834 33.139
#
# Call:  glm(formula = y ~ 1, data = z)
#
# Coefficients:
# (Intercept)
#      0.2488
#
# Degrees of Freedom: 9 Total (i.e. Null);  9 Residual
# Null Deviance:      10.29
# Residual Deviance: 10.29        AIC: 32.67