[R] Why do <none>s appear in the list of predictor variables in logistic regression using 'step' or 'stepAIC' function?

[Gabor Grothendieck]

On Tue, Jul 6, 2010 at 11:46 PM, Kiyoshi Sasaki <skiyoshi2001 at yahoo.com> wrote:
>
>
> Would anyone help me solve my problem with R, please? I am very new to R. I am doing logistic regression analysis on the presence/absence of salamanders using several predictor variables, as shown below. I have checked my data, but I didn't find any 'NA' or empty cells. When I used step() or stepAIC to select significant predictor variables, <none>s appear to places where predictor variables are listed (please see the bottom part of the codes I used and their output. Could anyone know what is going on? Just in cases, I copied the data I am using at the end of the output.
>

<none> refers to the line for which no variable is dropped and
therefore that line shows what the AIC is as you entered that step.
Each other line shows what the AIC is for dropping the indicated
variable so that if the variable is on a line above the <none> line
then dropping that variable lowers the AIC and if the variable is on a
line below the <none> line then dropping it increases the AIC.

<none> is not related to missing values.