[R] dropterm() in MASS

[Patrick Connolly]

To compare two different models, I've compared the result of using
dropterm() on both.


Single term deletions

Model:
growth ~ days + I(days^0.5)
            Df Sum of Sq     RSS     AIC
<none>                    2.8750 -0.2290
days         1    4.8594  7.7344  4.6984
I(days^0.5)  1    0.0234  2.8984 -2.1722

AND

Single term deletions

Model:
growth ~ days + I(days^2)
          Df Sum of Sq     RSS     AIC
<none>                  2.8750 -0.2290
days       1    5.8338  8.7088  5.5290
I(days^2)  1    0.0234  2.8984 -2.1722

I understood that the AIC column was the change in AIC when that term
was dropped from the model.

QUESTION: 

Why would (should) the effect of dropping quite different terms be
exactly the same?  It's consistent with all the data of this type I've
tried it on.

best

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