# RMS Describing, Resampling, Validating, and Simplifying the Model

**URL:** <https://discourse.datamethods.org/t/rms-describing-resampling-validating-and-simplifying-the-model/4790>\
**Category:** modeling strategy\
**Tags:** graphics, bootstrap, validation\
**Created:** [September 14, 2021, 7:24pm UTC](https://discourse.datamethods.org/t/rms-describing-resampling-validating-and-simplifying-the-model/4790 "2021-09-14T19:24:11Z")\
**Posts on this page:** 1\
**Showing post:** 158

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**Author:** ![Marc\_Vila\_Forteza](https://discourse.datamethods.org/user_avatar/discourse.datamethods.org/marc_vila_forteza/32/4428_2.png) [@Marc\_Vila\_Forteza](https://discourse.datamethods.org/u/Marc_Vila_Forteza)\
**Post date:** [March 21, 2024, 1:30pm UTC](https://discourse.datamethods.org/t/rms-describing-resampling-validating-and-simplifying-the-model/4790/158 "2024-03-21T13:30:22Z")

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Hi,  
this post comes from this other one that was posted in the wrong place:

[https://discourse.datamethods.org/t/validate-and-calibrate-cox-model-on-dataset-with-a-categorical-covariate-with-few-observations/8767?u=marc\_vila\_forteza](https://discourse.datamethods.org/t/validate-and-calibrate-cox-model-on-dataset-with-a-categorical-covariate-with-few-observations/8767?u=marc_vila_forteza)

Professor Harrell’s answer:  
You may have to combine infrequent levels. `Hmisc::combine.levels` can help with that. Or use the `group=` argument to `calibrate` and `validate` to do balanced bootstrap sampling. Specify something like `group=mydataset$FLUID2` .

→ Thank you for your quick reply. I have another question with regards the use of `group=`argument. Is it possible to use it for grouping more than one covariate at the same time?

I mean to do something similar to ‘group=c(mydataset$FLUID2,mydataset$TYPE)’.  
note: I tested this way to use a vector to specify more than one covariate but it didn’t work.

Thank you

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