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
--
*************************************************************
___ Patrick Connolly
{~._.~} HortResearch Great minds discuss ideas;
_( Y )_ Mt Albert Average minds discuss events;
(:_~*~_:) Auckland Small minds discuss people.
(_)-(_) New Zealand .... Anon
Ph: +64-9 815 4200 x 7188
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Dear Patrick, At 11:51 AM 4/29/2002 +1200, Patrick Connolly wrote:>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.Notice that the residual sums of squares for the two models are identical, so the peculiar result here pertains not just to dropping a term but also to the general fit of the model. What does the variable days look like? Are there just three different values of this variable? I hope that this helps, John ----------------------------------------------------- John Fox Department of Sociology McMaster University Hamilton, Ontario, Canada L8S 4M4 email: jfox at mcmaster.ca phone: 905-525-9140x23604 web: www.socsci.mcmaster.ca/jfox ----------------------------------------------------- -.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.- r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html Send "info", "help", or "[un]subscribe" (in the "body", not the subject !) To: r-help-request at stat.math.ethz.ch _._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._
I asked a question about dropterm():
Thanks to John Fox, I see it has to do with the inadequacies of the
model I was attempting to use and not to do with stepwise.
best
--
*************************************************************
___ Patrick Connolly
{~._.~} HortResearch Great minds discuss ideas;
_( Y )_ Mt Albert Average minds discuss events;
(:_~*~_:) Auckland Small minds discuss people.
(_)-(_) New Zealand .... Anon
Ph: +64-9 815 4200 x 7188
*************************************************************
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