Dear R users,
For the purpose of validating a prediction model using validate() from the rms
package, I am running into some trouble with using the fastbw() function
breaking up natural groups of variables.
Is there any way I can specify to keep certain variable together? In
particular, if interactions are included I would also like to keep the main
effects in the model.
Another example is a group of comorbidity variables that I would like to be
assessed all together and not as separate components as this makes more sense
from the perspective of interpretation.
The Force = ... option seems to only work for pre-specified variables and does
not solve my problem. In the case of the comorbidites, the variables are not
mutually exclusive, so I would be very difficult to recode them as a factor
variable with multiple levels.
Examples:
fit1 <-lrm(outcome~age + gender + med_a + med_b + med_a:med_b, x = TRUE, y =
TRUE, data = example)
fastbw(fit1, type = "residual).
--> fastbw may drop med_a and keep the med_a:med_b interaction instead.
fit2 <-lrm(outcome~age + gender + comorb_a + comorb_b + comorb_c + other
predictors, x = TRUE, y = TRUE, data = example)
fastbw(fit2, type = "residual).
--> fastbw may drop some of the comorbidities, but I would like to consider
them as one group.
Any help is much appreciated!
Maaike
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