Preface to First Edition - lib

ANALYSIS USING R 209R> plot(GBSG2_zph, var = "age")Beta(t) for age−0.6 −0.4 −0.2 0.0 0.2 0.4270 440 560 770 1100 1400 1800 2300TimeFigure 11.5Estimated regression coefficient for age depending on time for theGBSG2 data.tgrade.Q 3.22e-03 3.14e-03 0.95530pnodes 5.84e-02 5.98e-01 0.43941progrec 5.65e-02 1.20e+00 0.27351estrec 5.46e-02 1.03e+00 0.30967GLOBAL NA 2.27e+01 0.00695There seems to be some evidence of time-varying effects, especially for age andtumour grading. A graphical representation of the estimated regression coefficientover time is shown in Figure 11.5. We refer to Therneau and Grambsch(2000) for a detailed theoretical description of these topics.Martingale residuals as computed by the residuals method applied tocoxph objects can be used to check the model fit. When evaluated at thetrue regression coefficient the expectation of the martingale residuals is zero.Thus, one way to check for systematic deviations is an inspection of scatter-© 2010 by Taylor and Francis Group, LLC