Document Type : Research Articles
Department of Biostatistics, Faculty of Paramedical Sciences, Shahid Beheshti University of Medical Sciences, Qods Square, Darband Street, Tehran, Iran.
Pysiotherapy Research Center, Department of Biostatistics, Faculty of Paramedical Sciences, Shahid Beheshti University of Medical Sciences, Tehran, Iran.
Background: In its standard form, the parametric survival model assumes that the shape parameter is constant and the scaling parameter is not. This article focuses on how a model with a non-constant shape parameter could make differences in oncology studies and lead to more precise results. Materials and Methods: Online data for part of a large clinical trial conducted by the Radiation Oncology Group in the United States available online on UMass Amherst`s website were employed. The full study included patients with squamous cell carcinoma from fifteen sites in the mouth and throat, although only data on three sites in the oropharynx reported by the six largest institutions were considered here. To identify clinical, pathological and biological characteristics of patients which might have had an effect on their survival, we compared Weibull distributions once with a constant shape parameter and again with a non-constant shape parameter. Analyzes were performed using SAS university edition. The level of significance was set at P ≤ 0.05. Results: Based on the model with a constant shape parameter only the patient status was identified as a risk factor and the AIC of this model was 2152.4, but based on the model with a non-constant shape parameter, sex, patient status, stage of the tumor and the institute at which the patient had been treated were significant, with an AIC of 2150.1. Conclusion: On the basis of the AIC, the second model with a non-constant shape parameter was suggested to be more accurate for identifying risk factors, leading to more precise results.