A fundamental aspect of informed consent is that the patient fully understands the potential consequences of the suggested treatment, but new research suggests that this may not always be the case as patients often misinterpret the supplied probability information.

A questionnaire was administered to 42 inpatients (median age 52 years) who were assessed over a four month period. Respondees indicated their interpretation of fractional (eg 1 in 5) and percentage (eg 20%) probabilities using a crowd figure pictogram. The responses were then compared against a group of 50 older inpatients (median age 82 years) who had completed an identical questionnaire previously. The results showed that both groups made a high rate of errors in interpreting fractional probabilities except when expressed as 1 in 100. The use of different expressions of probabilities also proved problematic: 16% of subjects thought 1 in 5 and 5% were identical and 27% believed a risk of 1 in 20 was the same as 20%.

Although the younger group generally performed better than their older counterparts, pictorial representation of probabilities was well understood by both groups, leading the authors to suggest that the use of such visual aids should be introduced into clinical practice to ensure that patients understand the potential risks - and benefits - of suggested treatments.