How do parents of babies interpret qualitative expressions of probability?

One hundred mothers and 50 medical students and doctors were interviewed to examine their understanding of eight common probability expressions as they are used in statements of the kind that doctors regularly make to mothers of babies. Respondents were asked to translate each term into a number on a scale from 0 to 10. There was a wide range of interpretation of the expressions, and significant differences were found between the mothers and medical group for seven of the expressions. Most mothers expressed a preference for receiving information in numerical terms. We advise that verbal probability expressions should be restricted, and more information presented in numerical terms.