This example illustrates how to fit stratified Weibull models by using the STRATA statement.

The following data set contains survival times for 36 patients who were diagnosed with a malignant kidney tumor. Most of the patients received the therapy of nephrectomy (removal of all or part of the kidney).

data hyper; input nephrectomy age time status @@; datalines; 0 1 9 1 0 1 6 1 0 1 21 1 0 2 15 1 0 2 8 1 0 2 17 1 0 3 12 1 1 1 104 0 1 1 9 1 1 1 56 1 1 1 35 1 1 1 52 1 1 1 68 1 1 1 77 0 1 1 84 1 1 1 8 1 1 1 38 1 1 1 72 1 1 1 36 1 1 1 48 1 1 1 26 1 1 1 108 1 1 1 5 1 1 2 108 0 1 2 26 1 1 2 14 1 1 2 115 1 1 2 52 1 1 2 5 0 1 2 18 1 1 2 36 1 1 2 9 1 1 3 10 1 1 3 9 1 1 3 18 1 1 3 6 1 ;

The following statements convert the censored times into intervals:

data hyper; set hyper; left = time; if status = 0 then right = .; else right = time; run;

The following statements fit a stratified Weibull proportional hazards model:

ods graphics on; proc icphreg data=hyper plot(timerange=(0,125))=surv; class Age(desc); strata Nephrectomy; model (Left, Right) = Age / basehaz=splines(df=1); run;

The "Cubic Splines Parameters" table, shown in Output 63.3.1, contains the parameters for the cubic splines. Because DF=1, it is equivalent to a Weibull distribution.

Output 63.3.1: Baseline Hazard Parameters

The "Fit Statistics" table, shown in Output 63.3.2, contains the fit statistics.

Output 63.3.2: Model Fit Statistics

The table of parameter estimates is displayed in Output 63.3.3.

Output 63.3.3: Model Parameter Estimates

Analysis of Maximum Likelihood Parameter Estimates | ||||||||
---|---|---|---|---|---|---|---|---|

Effect | age | DF | Estimate | Standard Error |
95% Confidence Limits | Chi-Square | Pr > ChiSq | |

Coef1 | 1 | -9.2750 | 2.8327 | -14.8269 | -3.7231 | |||

Coef2 | 1 | 3.3252 | 0.9918 | 1.3814 | 5.2690 | |||

Coef3 | 1 | -5.2408 | 0.8785 | -6.9627 | -3.5188 | |||

Coef4 | 1 | 1.2861 | 0.2013 | 0.8915 | 1.6806 | |||

age | 3 | 1 | 1.8005 | 0.5966 | 0.6312 | 2.9697 | 9.11 | 0.0025 |

age | 2 | 1 | 0.1272 | 0.4017 | -0.6602 | 0.9146 | 0.10 | 0.7515 |

age | 1 | 0 | 0.0000 |

Output 63.3.4 and Output 63.3.5 show the predicted survival curves for the two strata at the reference level.

Output 63.3.4: Estimated Survival Curve at the Reference Level for Stratum 1

Output 63.3.5: Estimated Survival Curve at the Reference Level for Stratum 2