Statistical illiteracy undermines informed shared decision making.

Shared decision making relies on the exchange of information between the physician and the patient and the involvement of both patient and physician in making the decision. Informed shared decision making thus requires that patients and doctors understand the benefits and harms of different treatment options. This, however, is severely undermined by what we call collective statistical illiteracy. Both patients and physicians have difficulties to understand the meaning of numbers so that an effective risk communication cannot take place. Risk communication based on misunderstandings, however, renders the "informed" in informed shared decision making obsolete. We show that the problem of statistical illiteracy can largely be solved by changing the representation of statistical information. Insight can be achieved by communicating risks in absolute, not relative terms; by using a frequentist formulation, which makes the reference class clear instead of communicating single event probabilities; and by communicating natural frequencies instead of conditional probabilities.