The following is a simple example with a poor solution that shows my
difficulties with scope. The function /logit.test /has 3 arguments:
/model.start,/
an initial model; /model.finish/, an all-inclusive model, /my.data/, a
dataset, in
this case trivial.
There are 2 function calls in l/ogit.test,/ first to /glm/ to get an
initial fit (local variable
/logit/) using /model.start;/ then a call to /stepAIC/ using the initial
fit.
I had thought that l/ogit/ would carry along the dataset, /test.data/
for use in
the call to /stepAIC/. Instead it complains that it cannot find
/my.data/. It seems
to have found the name, not the value in the /call/ attribute.
I fixed the problem by creating a global variable /my.data/ within the
function,
naming it l/ogit.test.global,/ not a very good solution.
My question is: How do I handle scope correctly?
Thanks,
Terry Ireland
Script of Experiment:
> library(MASS)
> test.data
Approve Score OldScore
1 Yes 1 12
2 Yes 3 10
3 Yes 5 8
4 No 4 9
5 No 6 7
6 No 8 5
> test.start
Approve ~ 1
> test.finish
Approve ~ Score + OldScore
> logit.test
function(model.start,model.finish,my.data)
{
logit <- glm(model.start,binomial(link =
"logit"),my.data,control=glm.control(epsilon=0.0001),x=TRUE,y=TRUE);
Table <- stepAIC(logit,scope=model.finish,direction="both",trace=1,k=2);
}
> logit.test.global
function(model.start,model.finish,my.data)
{
my.data <<- my.data
logit <- glm(model.start,binomial(link =
"logit"),my.data,control=glm.control(epsilon=0.0001),x=TRUE,y=TRUE);
Table <- stepAIC(logit,scope=model.finish,direction="both",trace=1,k=2);
}
> logit.test(test.start,test.finish,test.data)
Start: AIC=10.32
Approve ~ 1
Error in inherits(x, "data.frame") : object "my.data" not found
> logit.test.global(test.start,test.finish,test.data)
Start: AIC=10.32
Approve ~ 1
Df Deviance AIC
+ Score 1 4.8089 8.8089
+ OldScore 1 4.8089 8.8089
<none> 8.3178 10.3178
Step: AIC=8.81
Approve ~ Score
Df Deviance AIC
<none> 4.8089 8.8089
- Score 1 8.3178 10.3178
>
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