RE: [R] question about about the drop1

From: <Bill.Venables_at_csiro.au>
Date: Sat 23 Apr 2005 - 19:10:57 EST


Ronggui asks:

: -----Original Message-----
: From: r-help-bounces@stat.math.ethz.ch
: [mailto:r-help-bounces@stat.math.ethz.ch] On Behalf Of ronggui
: Sent: Saturday, 23 April 2005 5:41 PM
: To: r help
: Subject: [R] question about about the drop1
:
:
: the data is :
: > table.8.3<-data.frame(expand.grid(
: marijuana=factor(c("Yes","No"),levels=c("No","Yes")),
: cigarette=factor(c("Yes","No"),levels=c("No","Yes")),
: alcohol=factor(c("Yes","No"),levels=c("No","Yes"))),
: count=c(911,538,44,456,3,43,2,279))
:
: > fit3<-glm(count ~ .^3, poisson, table.8.3)
: > sumary(fit3)
: ...
: Residual deviance: -1.5543e-15 on 0 degrees of freedom
: AIC: 65.043
:
: > drop1(fit3, .~., test="Chisq")

Putting in the dummy formula, ". ~ .", will force inappropriate terms (marginal terms) to be dropped from the model.

: Single term deletions
:
: Model:
: count ~ (marijuana + cigarette + alcohol)^3
: Df Deviance AIC LRT Pr(Chi)
: <none> -1.554e-15 65.04
: marijuana 1 365.78 428.83 365.78 < 2.2e-16 ***
: .... ~~~~~~~~~~~~~~
: marijuana:cigarette:alcohol 1 0.37 63.42 0.37 0.54084
: ~~~~~~~~~~~~~
:
: > update(fit3,.~.-marijuana )
: ...
: Residual Deviance: -1.188e-13 AIC: 65.04
: ~~~~~~~~~~~~~~~~~~~~~~~~~

Yes. This is the same as the full model, indicating that the update has not changed the model at all. You still have higher order interactions in the model so the main effect term has to be left in (or it's equivalent).                

:
: >update(fit3,.~.-marijuana:cigarette:alcohol )
: ...
: Residual Deviance: 0.374 AIC: 63.42
: ~~~~~~~~~~~~~~~~~~~~~~~~~

This update has removed the only non-marginal term from the model and so has made a change. This is why you are seeing the same deviance and AIC here as you saw with drop1. (This line from drop1 was the only appropriate one to be shown.)

:
: i find 0.374 and 63.42 are equal the result from drop1,but
: "Residual Deviance: -1.188e-13 AIC: 65.04 " is not.
:
: so i wonder why? what does the drop1 work exactly?

If you tried

drop1(fit3, test = "Chisq")

at the initial stage you would have only seen the three-way interaction. The other terms you are seeing with your inappropriate invocation of drop1 are not what you think they are.

:
: thank you !
:
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R-help@stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Sat Apr 23 19:15:25 2005

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