title: [Biases in the study of prognostic factors].
abstract: The main objective is to detail the main biases in the study of prognostic factors. Confounding bias is illustrated with social class, a prognostic factor still discussed. Within selection bias several cases are commented: response bias, specially frequent when the patients of a clinical trial are used; the shortcomings in the formation of an inception cohort; the fallacy of Neyman (bias due to the duration of disease) when the study begins with a cross-sectional study; the selection bias in the treatment of survivors for the different treatment opportunity of those living longer; the bias due to the inclusion of heterogeneous diagnostic groups; and the selection bias due to differential information losses and the use of statistical multivariate procedures. Within the biases during follow-up, an empiric rule to value the impact of the number of losses is given. In information bias the Will Rogers' phenomenon and the usefulness of clinical databases are discussed. Lastly, a recommendation against the use of cutoff points yielded by bivariate analyses to select the variable to be included in multivariate analysis is given.
authors.affiliation: Catedrático de Medicina Preventiva y Salud Pública, Facultad de Medicina, Universidad de Cantabria, Santander. delgadom@medi.unican.es
authors.forename: M
authors.initials: M
authors.lastname: Delgado-Rodríguez
mesh_terms: D015982:Bias; D016208:Databases, Factual; D005500:Follow-Up Studies; D006801:Humans; D010349:Patient Compliance; D018579:Patient Selection; D011379:Prognosis; D015203:Reproducibility of Results; D016312:Treatment Refusal