Hi,
I am trying to build an ordinal regression model using polr (from the
MASS package). In order to construct an initial model (without an error
aborting it) in my setting, I must pass in a "start" parameter. I would
then like to use the "step" function to remove unnecessary variables
from the model. However, this fails with the error message:
> mod1 <- step(model)
Start: AIC=42
PathCR ~ Cluster + [[stuff omitted]]
Error in polr(formula = PathCR ~ [[stuff omitted]] :
'start' is not of the correct length
The underlying problem appears to be that "step" calls "drop1" which
calls "update" on the formula with an omitted term. The "update" fails
with the same error message:
> update(model, ~ . - Cluster)
Error in polr(formula = PathCR ~ [[stuff omitted]] :
'start' is not of the correct length
Since "update" extracts the initial function call from the model, it
apparently passes the "start" parameters along to "polr" to refit the
model. Since one variable has been dropped, there are now too many
parameters in the "start" parameter and the "update" fails.
Does anyone have a way around this difficulty? (For an individual
update, I could probably hack the "call" object inside the existing
model, but I really don't see how to do that using 'step".)
Thanks,
Kevin
> sessionInfo()
R version 2.10.0 (2009-10-26)
i386-pc-mingw32
locale:
[1] LC_COLLATE=English_United States.1252
[2] LC_CTYPE=English_United States.1252
[3] LC_MONETARY=English_United States.1252
[4] LC_NUMERIC=C
[5] LC_TIME=English_United States.1252
attached base packages:
[1] stats graphics grDevices utils datasets methods base
other attached packages:
[1] MASS_7.3-4
loaded via a namespace (and not attached):
[1] tools_2.10.0
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