10.8 Cox proportional hazards regression

The Cox proportional hazards model is a regression model similar to those we have already dealt with. It is commonly used to investigate the association between the time to an event (such as death) and a set of explanatory variables.

Cox proportional hazards regression can be performed using survival::coxph() or the all-in-one finalfit() function. The latter produces a table containing counts (proportions) for factors, mean (SD) for continuous variables and a univariable and multivariable CPH regression.

10.8.1 coxph()

CPH using the coxph() function produces a similar output to lm() and glm(), so it should be familiar to you now. It can be passed to summary() as below, and also to broom::tidy() if you want to get the results into a tibble.

## Call:
## coxph(formula = Surv(time, status_os) ~ age + sex + thickness + 
##     ulcer, data = melanoma)
## 
##   n= 205, number of events= 71 
## 
##               coef exp(coef) se(coef)     z Pr(>|z|)    
## age       0.021831  1.022071 0.007752 2.816 0.004857 ** 
## sexMale   0.413460  1.512040 0.240132 1.722 0.085105 .  
## thickness 0.099467  1.104582 0.034455 2.887 0.003891 ** 
## ulcerYes  0.952083  2.591100 0.267966 3.553 0.000381 ***
## ---
## Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
## 
##           exp(coef) exp(-coef) lower .95 upper .95
## age           1.022     0.9784    1.0067     1.038
## sexMale       1.512     0.6614    0.9444     2.421
## thickness     1.105     0.9053    1.0325     1.182
## ulcerYes      2.591     0.3859    1.5325     4.381
## 
## Concordance= 0.739  (se = 0.03 )
## Likelihood ratio test= 47.89  on 4 df,   p=1e-09
## Wald test            = 46.72  on 4 df,   p=2e-09
## Score (logrank) test = 52.77  on 4 df,   p=1e-10

The output shows the number of patients and the number of events. The coefficient can be exponentiated and interpreted as a hazard ratio, exp(coef). Helpfully, 95% confidence intervals are also provided.

A hazard is the term given to the rate at which events happen. The probability that an event will happen over a period of time is the hazard multiplied by the time interval. An assumption of CPH is that hazards are constant over time (see below).

For a given predictor then, the hazard in one group (say males) would be expected to be a constant proportion of the hazard in another group (say female). The ratio of these hazards is unsurprisingly, the hazard ratio.

The hazard ratio differs from the relative risk and odds ratio. The hazard ratio represents the difference in the risk of an event at any given time, whereas the relative risk or odds ratio usually represent the cumulative risk over a period of time.

10.8.2 finalfit()

Alternatively, the a CPH regression can be run with finalfit functions. This is convenient for model fitting, exploration and the export of results.

The labelling of the final table can be adjusted as desired.

TABLE 10.1: Univariable and multivariable Cox Proportional Hazards: Overall survival following surgery for melanoma by patient and tumour variables (tidied).
Overall survival HR (univariable) HR (multivariable)
Age (years) Mean (SD) 52.5 (16.7) 1.03 (1.01-1.05, p<0.001) 1.02 (1.01-1.04, p=0.005)
Sex Female 126 (100.0)
Male 79 (100.0) 1.93 (1.21-3.07, p=0.006) 1.51 (0.94-2.42, p=0.085)
Tumour thickness (mm) Mean (SD) 2.9 (3.0) 1.16 (1.10-1.23, p<0.001) 1.10 (1.03-1.18, p=0.004)
Ulcerated tumour No 115 (100.0)
Yes 90 (100.0) 3.52 (2.14-5.80, p<0.001) 2.59 (1.53-4.38, p<0.001)

10.8.3 Reduced model

If you are using a backwards selection approach or similar, a reduced model can be directly specified and compared. The full model can be kept or dropped.

TABLE 10.2: Cox Proportional Hazards: Overall survival following surgery for melanoma with reduced model.
Dependent: Surv(time, status_os) all HR (univariable) HR (multivariable) HR (multivariable reduced)
Age (years) Mean (SD) 52.5 (16.7) 1.03 (1.01-1.05, p<0.001) 1.02 (1.01-1.04, p=0.005) 1.02 (1.01-1.04, p=0.003)
Sex Female 126 (100.0)
Male 79 (100.0) 1.93 (1.21-3.07, p=0.006) 1.51 (0.94-2.42, p=0.085)
Tumour thickness (mm) Mean (SD) 2.9 (3.0) 1.16 (1.10-1.23, p<0.001) 1.10 (1.03-1.18, p=0.004) 1.10 (1.03-1.18, p=0.003)
Ulcerated tumour No 115 (100.0)
Yes 90 (100.0) 3.52 (2.14-5.80, p<0.001) 2.59 (1.53-4.38, p<0.001) 2.72 (1.61-4.57, p<0.001)

10.8.4 Testing for proportional hazards

An assumption of CPH regression is that the hazard (think risk) associated with a particular variable does not change over time. For example, is the magnitude of the increase in risk of death associated with tumour ulceration the same in the early post-operative period as it is in later years?

The cox.zph() function from the survival package allows us to test this assumption for each variable. The plot of scaled Schoenfeld residuals should be a horizontal line. The included hypothesis test identifies whether the gradient differs from zero for each variable. No variable significantly differs from zero at the 5% significance level.

##               rho  chisq      p
## age        0.1633 2.4544 0.1172
## sexMale   -0.0781 0.4473 0.5036
## thickness -0.1493 1.3492 0.2454
## ulcerYes  -0.2044 2.8256 0.0928
## year       0.0195 0.0284 0.8663
## GLOBAL         NA 8.4695 0.1322

10.8.5 Stratified models

One approach to dealing with a violation of the proportional hazards assumption is to stratify by that variable. Including a strata() term will result in a separate baseline hazard function being fit for each level in the stratification variable. It will be no longer possible to make direct inference on the effect associated with that variable.

This can be incorporated directly into the explanatory variable list.

TABLE 10.3: Cox Proportional Hazards: Overall survival following surgery for melanoma stratified by year of surgery.
Dependent: Surv(time, status_os) all HR (univariable) HR (multivariable)
Age (years) Mean (SD) 52.5 (16.7) 1.03 (1.01-1.05, p<0.001) 1.03 (1.01-1.05, p=0.002)
Sex Female 126 (100.0)
Male 79 (100.0) 1.93 (1.21-3.07, p=0.006) 1.75 (1.06-2.87, p=0.027)
Ulcerated tumour No 115 (100.0)
Yes 90 (100.0) 3.52 (2.14-5.80, p<0.001) 2.61 (1.47-4.63, p=0.001)
Tumour thickness (mm) Mean (SD) 2.9 (3.0) 1.16 (1.10-1.23, p<0.001) 1.08 (1.01-1.16, p=0.027)
strata(year)

10.8.6 Correlated groups of observations

As a general rule, you should always try to account for any higher structure in your data within the model. For instance, patients may be clustered within particular hospitals.

There are two broad approaches to dealing with correlated groups of observations.

Adding a cluster() term is similar to a generalised estimating equations (GEE) approach (something we’re not covering in this book). Here, a standard CPH model is fitted but the standard errors of the estimated hazard ratios are adjusted to account for correlations.

A frailty() term implies a mixed effects model, where specific random effects term(s) are directly incorporated into the model.

Both approaches achieve the same goal in different ways. Volumes have been written on GEE vs mixed effects models and we won’t rehearse them in this introductory book. We favour the latter approach because of its flexibility and our preference for mixed effects modelling in generalised linear modelling. Note cluster() and frailty() terms cannot be combined in the same model.

##  Dependent: Surv(time, status_os)                   all
##                       Age (years) Mean (SD) 52.5 (16.7)
##                               Sex    Female 126 (100.0)
##                                        Male  79 (100.0)
##             Tumour thickness (mm) Mean (SD)   2.9 (3.0)
##                  Ulcerated tumour        No 115 (100.0)
##                                         Yes  90 (100.0)
##              cluster(hospital_id)                      
##           HR (univariable)        HR (multivariable)
##  1.03 (1.01-1.05, p<0.001) 1.02 (1.00-1.04, p=0.016)
##                          -                         -
##  1.93 (1.21-3.07, p=0.006) 1.51 (1.10-2.08, p=0.011)
##  1.16 (1.10-1.23, p<0.001) 1.10 (1.04-1.17, p<0.001)
##                          -                         -
##  3.52 (2.14-5.80, p<0.001) 2.59 (1.61-4.16, p<0.001)
##                          -                         -
TABLE 10.4: Cox Proportional Hazards: Overall survival following surgery for melanoma (frailty model)
Dependent: Surv(time, status_os) all HR (univariable) HR (multivariable)
Age (years) Mean (SD) 52.5 (16.7) 1.03 (1.01-1.05, p<0.001) 1.02 (1.01-1.04, p=0.005)
Sex Female 126 (100.0)
Male 79 (100.0) 1.93 (1.21-3.07, p=0.006) 1.51 (0.94-2.42, p=0.085)
Tumour thickness (mm) Mean (SD) 2.9 (3.0) 1.16 (1.10-1.23, p<0.001) 1.10 (1.03-1.18, p=0.004)
Ulcerated tumour No 115 (100.0)
Yes 90 (100.0) 3.52 (2.14-5.80, p<0.001) 2.59 (1.53-4.38, p<0.001)
frailty(hospital_id)

The frailty() method here is being superseded by the coxme package, and we look forward to incorporating this in the future.

10.8.7 Hazard ratio plot

A plot of any of the above models can be produced using the hr_plot() function.