BUILT-IN MATHEMATICA SYMBOL

# StrataVariables

StrataVariables
is an option for fitting functions such as CoxModelFit that specifies the variables on which the model should be stratified.

## DetailsDetails

• Strata variables are used to divide a non-homogenous population into homogenous subgroups. Stratification is often useful in proportional hazards modeling where the assumption of proportional hazards between subgroups may be suspect.
• Possible settings for StrataVariables include:
•  None treat no variables as strata {v1,v2,...} treat the variables as strata
• Models with a strata variable v with p levels effectively groups the input data into p groups corresponding to the .
• With multiple strata variables , ..., with levels, the input data are grouped into at most groups, dropping empty subgroups.

## ExamplesExamplesopen allclose all

### Basic Examples (2)Basic Examples (2)

Define some data:

Fit the data, treating the second variable as a strata variable:

 Out[2]=

One stratum is created for each level in the data:

 Out[3]=

A parameter summary table:

 Out[4]=

Fit a model with more than one strata variable:

 Out[2]=

All non-empty combinations of the strata variables are used:

 Out[3]=

Baseline survival rates for the strata:

 Out[4]=

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