  Classically, risk is characterized by a point value probability indicating
the likelihood of occurrence of an adverse effect. However, there are domains
where the attainability of objective numerical risk characterizations is
increasingly being questioned. This paper reviews the arguments in favour of
extending classical techniques of risk assessment to incorporate meaningful
qualitative and weak quantitative risk characterizations. A technique in which
linguistic uncertainty terms are defined in terms of patterns of argument is
then proposed. The technique is demonstrated using a prototype computer-based
system for predicting the carcinogenic risk due to novel chemical compounds.
