Accurate diagnostic and prognostic probabilities are important for physicians and their patients because of their impact on decisions. For individual patient decision making, physicians have to rely on their own judgement. Research has shown that physicians' probability assessments are often not of high quality and that a variety of factors have a detrimental influence on their judgements. Using a formula or a model may produce superior assessments. Most treatment decisions involve uncertainty and value conflict in the sense that there is a trade off between the possible harms of alternative treatments and their benefits. The communication of diagnostic and prognostic probabilities to patients occurs largely in verbal terms, which can lead to ambiguity in communication. The precise words and context chosen for communication are very important and can have a major effect on patients' decisions.