# #StackBounty: #r #machine-learning #mathematical-statistics #survival Accelerated Failure Time Regression Performance (Survival Analysis)

### Bounty: 100

Issue is around high intercept with AFT Regression. Let me explain below:

Suppose you are modelling the time to an event via an Accelerated Failure Time Regression i.e. given survival time $$T$$, suppose we have observed values of covariates $$x_{i1}, …, x_{ip}$$ and possibly censored survival time $$t_i$$, then:
$$log(t_i) = beta_0 + beta_1 x_{i1} + … + beta_p x_{ip}+ sigma epsilon_i$$

Suppose we are looking at a Weibull AFT i.e. where $$epsilon_i$$ are IID according to a Gumbel Distribution (Extreme Value Type 1).

You are looking at the case of time varying covariates (assume just one for now) e.g. you have a dataset like the following example with a single time dependent covariate (TDC_1). Where Start is the enter time (period start) and End is the period end (exit time) and UNIT_ID is the ID for the entity in the study:

``````START END EVENT UNIT_ID TDC_1
0     1   0     1       0.1
1     2   0     1       0.2
2     3   0     1       0.3
...
19    20  1     1       1.9
0     1   0     2       0.1
1     2   0     2       0.2
2     3   0     2       0.3
...
19    20  1     2       1.9
``````

With the `aftreg` function from the `eha` library in R you can construct a Weibull AFT e.g.

``````model <- aftreg(Surv(START, END, EVENT) ~ TDC_1, dist="weibull", data=df, id=UNIT_ID, param='lifeExp')
``````

Calling `model.coefficients` gives:

``````             model.coefficients
TDC_1        -0.905
log(scale)    9.393
log(shape)    0.046
``````

The expected time to event when $$T$$ follows a Weibull distrubtion is given by:
$$E(T|X_i) = exp left( beta_0 + x_i beta_1 right)Gamma(1 + sigma) = exp left( 9.393 – 0.905*TDC_1 right)*0.98$$

As $$beta_0 = log(scale)$$ and $$sigma = frac{1}{exp(log(shape))}$$

My question is around these parameter estimates (in particular the the intercept term ($$beta_0 = log(scale)$$). No matter how I change the error term parameterisation e.g. if $$epsilon_i$$ are distributed normally (then $$T$$ lognormal) or if $$epsilon_i$$ ~ Logistic etc, the intercept is exceptionally high and appears not to be optimal in terms of minimising error on time to event.

For example if I manually subtract 2 from the intercept (9.393 – 2) I can reduce the root mean squared error on the time to event on the dataset fit:.

``````Intercept TIME_TO_EVENT_RMSE
9.393     776 days
7.393     97 days
``````

Here TIME_TO_EVENT_RMSE is calculated as (with a dataset that only contains non-censored events):

$$RMSE = sqrt{sum_{i}^{n} frac{(exp left( beta_0 + x_i beta_1 right)Gamma(1 + sigma) – t_i)^2}{n}}$$

For further illustration, suppose you model directly using exponential regression (i.e. linear regression and logging the target variable) with exactly the same dataset (only using non-censored events so the two are comparable). I know they are minimising different loss functions and aren’t directly comparable, but just for illustration purposes:

``````TIME_TO_EVENT UNIT_ID TDC_1
19             1      0.1
18             1      0.2
17             1      0.3
...
``````

Here we have:

$$E(T|X_i) = exp left( beta_0 + x_i beta_1 right) = exp left( 8.03 – 0.5*x_i right)$$

I know that AFT Regression is not directly minimising RMSE, and that with the AFT regression the TDC_1 coefficient magnitude is larger in addition to a larger intercept, however with the intercept as high as it is, the model isn’t particularly useful (significantly over-predicting the time to event).

Questions:

1. Has anyone experienced this before and have any advice on how to improve the AFT model?
2. Is there anyway to fix the scale with time varying covariates in AFTRegression?

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