Communicating probabilistic information to cancer patients: is there 'noise' on the line?

The objective was to examine the way that cancer patients translate verbal descriptors of probability into numerical estimates. A list of words commonly used on consent forms to describe the likelihood for benefits or risks of therapies was provided to 100 cancer patients. Two formats, paper/pencil or computer, were used to provide the list of words. Two methods, magnitude estimation and linear analogue scaling, were used to obtain probability estimates for each word. In addition, two scenarios were developed to study 'context effects' on numerical interpretations of verbal descriptions of probability. All patients provided numerical values for the words on two occasions, separated by one week, and two interviewers collected the data. Regardless of method or format, each word elicited widely variable numerical interpretations. An ANOVA model, including patient, word, interviewer, time, method and format, indicated that patient and interviewer produced major effects on probability estimates. Agreement between methods and across time was good. Paper/pencil and computer formats yielded similar results. Context effects did not appear to influence the numerical probabilities elicited by the 2 scenarios. It was concluded that, within this group of patients, there was no consensus about numerical meaning of a given word, and that interviewers can systematically influence numerical interpretations. There appears to be a great deal of 'noise' in this particular line of communication between patients and health professionals.