Abstract
This chapter presents the principles of uncertainty, imprecision, vagueness, incomplete and random data types, treatments and diagnostics principles in medicine. For example, a disease can manifest quite differently from a patient, doctor and social perspective. It is stated that knowledge-based medical treatment decision support systems will develop further with fuzzy logic evaluation due to uncertainty involvement. This chapter offers a series of recommendations on aspects of uncertainty in the medical sciences. The comparison of fuzzy and bivalent (crisp) logic and the fuzzy logic preference in medicine are explained with evidence. Fuzzy logic inference modeling with medical vague words is presented on a sample with four inputs (temperature, blood pressure, urine quality and heart rate), for diagnosing disease as an output estimate. Finally, the use of fuzzy logic in medicine is explained by a set of diseases classifications.