StepAIC stopping point

Question

I'm attempting to comprehend where StepAIC stops (). Does it stop using direction = "backward" if further term deletions no longer result in a lower model AIC? as an illustration:

fm-lm(mpg., data = mtcars)-require(MASS)-fit fm-step

AIC(fm, direction = 'reverse')
#
The last action halted at:
Step: Wt + qsec + am AIC=61.31 mpg

169 61.3 - am 1 26.2 195 63.9 - qsec 1 109.0 278 75.2 - wt 1 183.3 353 82.8 Df Sum of Sq RSS AIC
Does this imply that removing any of the terms wt, qsec, or am won't have any effect on the model AIC (all AICs = 61.31)?

Answer 1

Square RSS Df Sum AIC 169 61.3 - am 1 26.2 195 63.9

- qsec 1 109.0 278 75.2

- wt 1 183.3 353 82.8

If you remove any further words, this table will display the AIC, with the row representing the active model.

If you leave the model in its current state, the AIC is 61.3 and the extra terms have been eliminated. If you remove those phrases, each subsequent row displays the AIC (remove am, get AIC of 63.9, etc.). You are true that removing any of the remaining phrases will increase the AIC because you have reached the local minimum for the AIC.

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