stepAIC invalid scope argument

Adai,

The following works.Perhaps you should define your 'upper' and 'lower'
in the list as aov's, as you have done with your lo,hi and mid.

John

 stepAIC( mid, scope=list(upper = mid , lower = lo) )

Start:  AIC= -594.66 
 y ~ x2 + x3 

       Df Sum of Sq     RSS     AIC
- x2    1      0.11  548.56 -596.45
- x3    1      0.95  549.40 -594.93
<none>               548.45 -594.66

Step:  AIC= -596.45 
 y ~ x3 

Adaikalavan Ramasamy wrote ---

I am trying to replicate the first example from stepAIC from the MASS
package with my own dataset but am running into error. If someone can
point where I have gone wrong, I would appreciate it very much. 

Here is an example :

 set.seed(1)
 df   <- data.frame( x1=rnorm(1000), x2=rnorm(1000), x3=rnorm(1000) )
 df$y <- 0.5*df$x1 + rnorm(1000, mean=8, sd=0.5)
 # pairs(df); head(df)

 lo  <- aov( y ~ 1, data=df )
 hi  <- aov( y ~ .^2, data=df )
 mid <- aov( y ~ x2 + x3, data=df )

Running any of the following commands

 stepAIC( mid, scope=list(upper = ~x1 + x2 + x3 , lower = ~1) )
 stepAIC( mid, scope=list(upper = hi , lower = lo) )
 addterm( mid, ~ x1 + x2 + x3 )
 addterm( lo, hi )

gives the same error message : 
  Error in eval(expr, envir, enclos) : invalid second argumen