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Hình 6.11 cho cốt truyện xác suất của thời gian tồn tại của 234 bệnh nhân nam mắc bệnh bạch cầu lymphocytic mãn tính, trong đó trục ngang cho thời gian tồn tại về quy mô logarit và trục thẳng đứng là quy mô xác suất bình thường | 306 IDENTIFICATION OF PROGNOSTIC FACTORS RELATED TO SURVIVAL TIME Table 12.1 Results of a Proportional Hazards Regression Analysis of Data in Table 11.4 Covariate Regression Coefficient Standard Error p Value exp coefficient X1 age 1.01 0.46 0013 2.75 X cellularity 0.35 0.44 0.212 1.42 The 95 confidence intervals for br age and b2 cellularity are 1.01 1.96 0.46 or 0.11 1.91 and 0.35 1.96 0.44 or 0.51 1.21 respectively. Consequently the 95 confidence intervals for the relative risks are gO-11 gi-91 or 1.12 6.75 and e 51 e1 21 or 0.60 3.35 respectively. The small number of patients 30 may have contributed to the large standard errors of and b2 and consequently the wide confidence intervals. The lower bound of the confidence interval for age is only slightly above 1. This suggests that the importance of age should be interpreted carefully. In general if the number of subjects is small and the standard errors of the estimates are large the estimates may be unreliable. When the two covariates are considered simultaneously the risk for a patient with Xj 1 and x2 1 relative to patients with Xj 0 and x2 0 can be estimated. The relative risk is estimated as exp 1.01 0.35 3.90 for a patient who is over 50 years of age and whose cellularity is 100 compared to patients who are younger than 50 and whose cellularity is less than 100 . Using the same data set C AML.DAT defined in Example 11.3 the following SAS code can be used to obtain the results in Table 12.1. data w1 infile c aml.dat missover input t cens x1 x2 run proc phreg model t cens 0 x1 x2 rl run If BMDP 2L is used the following code is applicable. input file c aml.dat . print variables 4. format free. cova. variable names t cens x1 x2. form time t. regress status cens. response 1. covariates x1 x2. PARTIAL LIKELIHOOD FUNCTION FOR SURVIVAL TIMES 307 If SPSS is used the following code suffices. data list file c aml.dat free t cens x1 x2. coxreg t with x1 x2 status cens event 1 print all. Example 12.2 In a study Buzdar .
