[R] dropterm() in MASS

Patrick Connolly P.Connolly at hortresearch.co.nz
Mon Apr 29 01:51:50 CEST 2002

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

Single term deletions

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


Single term deletions

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.


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.


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