Ordinal classification in medical prognosis

Objectives: Medical prognosis is commonly expressed in terms of ordered outcome categories. This paper provides simple statistical procedures to judge whether the predictor variables reflect this natural ordering. Methods: The concept of stochastic ordering in logistic regression and discrimination models is applied to naturally ordered outcome scales in medical prognosis. Results: The ordering stage is assessed by a data-generated choice between ordered, partially ordered, and unordered models. The ordinal structure of the outcome is particularly taken into consideration in the construction of allocation rules and in the assessment of their performance. The specialized models are compared to the unordered model with respect to the classification efficiency in a clinical prognostic study. Conclusions: It is concluded that our approach offers more flexibility than the widely used cumulative-odds model and more stability than the multinomial logistic model. The procedure described in this paper is strongly recommended for practical applications to support medical decision-making.