title: Categorizing a prognostic variable: review of methods, code for easy implementation and applications to decision-making about cancer treatments.
abstract: Categorizing prognostic variables is essential for their use in clinical decision-making. Often a single cutpoint that stratifies patients into high-risk and low-risk categories is sought. These categories may be used for making treatment recommendations, determining study eligibility, or to control for varying patient prognoses in the design of a clinical trial. Methods used to categorize variables include: biological determination (most desirable but often unavailable); arbitrary selection of a cutpoint at the median value; graphical examination of the data for a threshold effect; and exploration of all observed values for the one which best separates the risk groups according to a chi-squared test. The last method, called the minimum p-value approach, involves multiple testing which inflates the type I error rates. Several methods for adjusting the inflated p-values have been proposed but remain infrequently used. Exploratory methods for categorization and the minimum p-value approach with its various p-value corrections are reviewed, and code for their easy implementation is provided. The combined use of these methods is recommended, and demonstrated in the context of two cancer-related examples which highlight a variety of the issues involved in the categorization of prognostic variables.
authors.affiliation: Division of Epidemiology and Biostatistics, Memorial Sloan-Kettering Cancer Center, 1275 York Avenue, New York, NY 10021, USA. mazumdar@biosta.mskcc.og
authors.forename: M
authors.initials: M
authors.lastname: Mazumdar
authors.forename: J R
authors.initials: JR
authors.lastname: Glassman
mesh_terms: D000970:Antineoplastic Agents / Q000008:administration & dosage; D016009:Chi-Square Distribution; D006801:Humans; D008223:Lymphoma / Q000188:drug therapy; D008297:Male; D015233:Models, Statistical; D009369:Neoplasms / Q000628:therapy*; D011336:Probability; D011379:Prognosis; D012189:Retrospective Studies; D012306:Risk; D018239:Seminoma / Q000188:drug therapy / Q000532:radiotherapy / Q000601:surgery; D016019:Survival Analysis; D013736:Testicular Neoplasms / Q000188:drug therapy / Q000532:radiotherapy / Q000601:surgery; D014057:Tomography, X-Ray Computed; D016896:Treatment Outcome
chemical_list: D000970:Antineoplastic Agents