__Title__

**tvc note** -- Splitting the data at failure times to reproduce the results
from **stcox** with the **tvc()** option

__Description__

This entry shows, by an example, an alternative way to produce the
results from **stcox** with the **tvc()** option. We will split the data at
failure times (using **stsplit**) and generate interaction terms manually.
In general, using **tvc()** is the most efficient way to fit a model with
time-varying covariates. However, doing the process manually allows the
use of some extra features.

__Remarks__

The **tvc()** option provides a quick and convenient way to model
time-varying covariates. A new predictor is added to the model for each
variable specified in **tvc()**. Each new predictor is equivalent to the
product of the corresponding variable with a function of the study time.
We will refer to these new predictors as interaction terms.

The Cox model is fit by evaluating the partial likelihood at each failure
time. We will need to generate new variables if we want to manually
include the above interaction terms in the model fit. Because the
interaction terms are a function of the study time, this will require us
to split the dataset at the failure times.

__Example__

Setup
**. webuse drugtr**

See the setting for survival data
**. stset**

Fit a Cox model with the following covariates: **drug**, **drug** as a
time-varying covariate, and **age** only as a time-varying covariate. Use
the square of study time in the interaction terms.

**. stcox drug, tvc(drug age) texp(_t^2)**

Now we'll produce the same results by using **stsplit**. This will take four
steps:

1. Generate a subject identifier variable if we don't already have one.
Make sure to include this variable in the **id()** option of **stset**.

**. generate id = _n**
**. streset, id(id)**

2. Use **stsplit** to split the data at the failure times.

**. stsplit, at(failures)**

3. Generate the variables representing the interaction terms.

**. generate agetvc = age*(_t^2)**
**. generate drugtvc = drug*(_t^2)**

4. Fit the Cox model, including the variable drug and the two variables
interacted with the study time.

**. stcox drug drugtvc agetvc**