Speakers |
Patrick Royston, Royal Postgraduate Medical School Peter Sasieni, Imperial Cancer Research Fund |

The Cox regression model attempts to quantify the relative hazard of an
event, say death, at varying times after some time origin, such as diagnosis
or initiation of treatment. The data consist of survival times, some of
which are censored if the study ended before an event was observed for every
patient, a censoring indicator variable which is 1 for events and 0 for
censored observations, and usually covariates
. The hazard of
death at time *t*, given the values of the covariates, is represented as
proportional to a baseline hazard function :

where is a vector of regression coefficients. In the estimation of the parameters, , the baseline hazard function does not need to be explicitly estimated. This flexibility is perhaps the strongest feature of the Cox model.

In most medical papers, the authors take the proportional hazards (PH) assumption for granted and make no attempt to check that it has not been violated in their study data. However, it is a strong assumption indeed. Suppose there was a hypothetical treatment which was quite effective in reducing the death rate from some condition during the first year after diagnosis, but whose effectiveness wore off rapidly after that, as the disease process reasserted itself. PH dictates that the treatment effect is constant on the relative hazard scale, so the estimated regression coefficient for treatment from the Cox model will actually represent some kind of time-averaged treatment effect. The important fact that the treatment effect is transient is undetectable within the standard Cox framework.

One approach to detecting non-PH is to extend the Cox model as follows:

Now is a function of follow-up
time, thus allowing transient and other time-related covariate effects to be
represented in an intuitive way. There are different ways of gaining an
impression of the nature of . Two
ways are described here. See Valsecchi et al. (1996) for further details and
examples. The first way is to impose a `window' on the data and estimate
locally within the window
according to a standard Cox model. The window is then shifted and the
process is repeated. It is convenient to define each window as containing a
fixed number of events (deaths), such as 10 × *k*. The local
estimates of the and 95%
confidence intervals, derived as ± 2 times the standard error, are then
plotted against the central time point of each window to give an impression
of the functional form. This approach is implemented in the ado-files
**stbtcalc** and **stbtplot**. The second way is through quantities
known as Schoenfeld residuals. The technical details are given by Gramsch
and Therneau (1994) and are not required here. Again a local estimate of
is obtained, but this time it is
essential to apply a smoother to get an impression of the functional form.
We use **running** (Sasieni 1995; Sasieni & Royston 1998). The
technique is implemented in the ado-files **stgtcalc** and
**stgtplot**. Alternatively, a more restricted smoother such as
**fracpoly** could be applied. Use of **fracpoly** (or other
parametric models, such as regression splines) has added value in that it
allows us to define, albeit heuristically, significance tests of non-PH.

Having identified that non-PH is present, the next step may be to try to model parametrically. It requires the dataset to be expanded in a particular way, and a Cox regression with time-varying covariates must be estimated. We won't go into this here.

In the talk, we will give a computer demonstration of the use of the new ado-files in detecting non-proportional hazards in a clinical trial dataset. Some details are given in the next section.

Myelomatosis is a cancer of the bone marrow which affects the immune system. One of the effects of the disease is the secretion by cancer cells of the protein -microglobulin (B2). A high serum concentration of B2 indicates advanced disease and is associated with a poor prognosis. The British Medical Research Council has carried out several controlled clinical trials of treatments for myelomatosis. We analyze data from the fourth and fifth trials (MacLennan et al. 1988). A total of 1087 patients were entered, of whom 571 had died by 913 days (about 2.5 years) after randomization, the latest time with complete follow up on all patients. We use Cox regression to analyze the relation between the B2 concentration at randomization and the log relative hazard of death.

Preliminary analysis reveals *k* = 4 covariates that are significant
(*P* < 0.01) predictors of the hazard of death in a multivariable
model. Although treatment effects were significant, the treatments in trials
4 and 5 were different and for simplicity we don't consider them here. The
covariates are the patient's **age**
(), and the blood concentrations of
-microglobulin (**sb2**,
), hemoglobin (**hb**,
) and **calcium**
(). The following table shows the
results of analysis using **stcox**.

. stset time censdata set name: myel45.dta id: -- (meaning each record a unique subject) entry time: -- (meaning all entered at time 0) exit time: time failure/censor: cens. stcox c2 age sb2 hb calciumfailure time: time failure/censor: cens Iteration 0: Log Likelihood = -5353.41

Cox regression -- entry time 0 No. of subjects = 1050 Log likelihood = -5282.3789 No. of failures = 854 chi2(4) = 142.06 Time at risk = 2659.268992 Prob > chi2 = 0.0000 ------------------------------------------------------------------------------ time | cens | Haz. Ratio Std. Err. z P>|z| [95% Conf. Interval] ---------+-------------------------------------------------------------------- sb2 | 1.028496 .003653 7.911 0.000 1.021361 1.03568 hb | .9932634 .0015708 -4.274 0.000 .9901894 .9963469 calcium | 1.363158 .1127263 3.746 0.000 1.159194 1.60301 age | 1.014327 .004299 3.356 0.001 1.005936 1.022788 ------------------------------------------------------------------------------

All the chosen covariates have effects significant at the 0.1% level.
Figure 1 shows the graphical output from the **stbtplot** command,
created using the following commands:

. stbtcalc c2 sb2 hb calcium age . stbtplot sb2 hb calcium age, xlabel(0,1,2,3,4,5,6,7) ylabel ci border

We have omitted results for **c2** from the plot to improve legibility.
Although there is some noise in each plot, the effect on the hazard is
fairly stable over time. The effects of **sb2**, **hb**, **age**
and **calcium** appear time-varying, the least so for **calcium**. The
effect of **sb2** seems to reach a peak after about 3–6 months and
dwindle thereafter.

Figure 2 shows the graphical output from the **stgtplot** command,
created using the following commands:

. stset time cens . stcox c2 age sb2 hb calcium . stgtcalc . stgtplot sb2 hb calcium age, xlabel(0,1,2,3,4,5,6,7) ylabel ci border

Notice that you must call **stcox** before **stgtcalc**, because
**stgtcalc** (unlike **stbtcalc**) is a post-estimation command. The
plots provide an estimate of up
to nearly 7 years of follow up—in Figure 1,
is estimated only up to *t*
= 5 years. Although the smoothing has not removed all the noise, the
impression again is that all the effects are time-varying. Qualitatively,
the two sets of plots agree quite well, though the effect size generally
seems larger with the **stbtcalc** approach. When in doubt we should
probably trust the results from **stbtcalc** more than those from
**stgtcalc**, since the **stbtcalc** estimation method is direct and
should not produce much bias.

We can confirm the subjective impressions from Figure 2 parametrically by
fitting first degree fractional polynomials to estimate
from the Grambsch–Therneau
(GT) residuals and applying a hypothesis test of no effect. The GT residuals
are quietly created by **stgtcalc** as new variables whose names begin
with the prefix **GT**. For example, the GT residuals for **sb2** are
stored in **GTsb2**. We would enter

. fracpoly regress GTsb2 time, degree(1) compare

The results show that there is a nonlinear effect of **time** on
**GTsb2**, since the deviance difference between the first degree
fractional polynomial model and the null model is 23.18. The deviance
difference is significant at *P* < 0.001 when compared with
chi-squared on 2 df. The first degree fractional polynomial, which has
estimated power 0.5, fits significantly better than a straight line
(*P* = 0.023). The same technique applied to **age**, **hb** and
**calcium** gives *P* values of 0.037, 0.45 and 0.13, showing
significant non-proportional effects of age but not of hemoglobin or
calcium.

- Grambsch, P. M. and T. Therneau. 1994.
- Proportional hazards tests and
diagnostics based on weighted residuals.
*Biometrika*81: 515–526.

- MacLennan, I. C. M., K. Kelly, R. A. Crockson, E. H. Coopler, J. Cuzick, and C. E. Chapman. 1988.
- Results of the MRC myelomatosis trials for patients
entered since 1980.
*Hematological Oncology*6: 145–158.

- Sasieni, P. 1995.
- sed9: Symmetric nearest neighbor linear smoothers.
*Stata Technical Bulletin*24: 10–14.

- Sasieni, P. and P. Royston. 1998.
- sed9.1: Pointwise confidence intervals
for running.
*Stata Technical Bulletin*41: 17–23.

- Valsecchi, M. G., D. Silvestri, and P. D. Sasieni. 1996.
- Evaluation of long
term survival: use of diagnostics and robust estimators with Cox's proportional
hazards models.
*Statistics in Medicine*15: 2763–2780.