In the first article in this series we looked at the dimension of risk. Expression of risk in terms of an unwanted outcome or event can be described with descriptions or distinctions based on both its quality or on its quantity. The probability can be described in qualitative terms such as rare or infrequent or expressed quantitatively such as 1 in 1000. What is important to acknowledge that patients differ in what they like.

For example, in the context of a study on an invasive diagnostic procedure, 467 patients varied in terms of the way the risk was told to them: 43 % preferred qualitative (verbal expressions) disclosure of information vs. 36% preferring quantitative. The exact number varies form study to study, but roughly it is about half and half for each type of disclosure. In terms of influenza vaccines for children, slightly more parents preferred quantitative information (60%). It is often interesting how we come to decisions without knowing the basic quantitative risk. For instance, ask yourself the question: What is the risk of death from swine flu compared to seasonal flu? Qualitative descriptions are often foremost in our minds and very few will be able to give quantitative information. To be honest, I think I’ve shot myself in the foot as I don’t know the answer to this question myself. But, my premise is correct, as words like rare and infrequent are what come to my mind.

The most useful strategy identified in a systematic review of effective formats for communicating probabilistic information, including 15 randomized controlled trials found for both written and verbal information, patients have a more accurate perception of risk if probabilistic information is presented as numbers rather than words. That means that it is all right to say words like rare or infrequent but without a quantitative number such as 1 in a million people find it difficult to correctly perceive the actual probability.

How to present qualitative information

Qualitative descriptions are appealing because they use common words that seem to be generally well understood. However, qualitative words, of which there are many, have no generally accepted anchoring at specific quantitative levels of frequency, despite efforts to promote such anchoring.

Data about actions sometimes bear little relation to the statistics. Saxe found no connection between the probabilities provided to couples during genetic counselling and the decisions they made. The use of verbal categories with scales of risk, such as very unlikely, was easier to use and represented better their true feelings.

Proposals have been made to standardise the language of risk with standardised terms for specified frequencies ("high" for risks 1 in less than 100 and "moderate" for between 1 in 100 and 1 in 1000). However, it’s all in the interpretation and patients would probably not understand such standardised terms consistently.

There are a number of simple recommendations worth incorporating based on this article:
In describing risk choose either:

1) numeric probabilities, or
2) numeric and qualitative information together

Qualitative descriptions should use common words that seem to be generally understood.

Next time you are met with a risk, whether presenting or reading about another scare story in the media, you might want to consider whether some of these issues outlined have been met. They probably won’t have.