Usage of words related to chance

  1. Are people consistent in associating words like "likely" with numerical probabilities like "80%"?
  2. How should a scientific consensus or dispute about a certain risk be communicated "in words" to the public?
  3. How do "ordinary people" actually use phrases like "unlucky" or "slim chance"?
  4. Do the different usages of different words -- "lucky" and "fortunate", for instance -- correspond to any conceptually interesting real difference, and if so can we elucidate that difference more precisely?
Note the careful phrasing: 1 and 3 are empirical questions, 2 and 4 are not.

1. Are people consistent in associating words with numerical probabilities?

This question has been the subject of a moderate amount of academic research, which I will not try to summarize beyond saying "reasonably consistent". See for instance this figure extracted from Measuring the vague meanings of probability terms in which people were asked to associate an interval of numerical values with words like "doubtful".

2. How should quantitative assessments of risk be communicated "in words" to the public?

This is a main focus of the website Understanding Uncertainty -- see for example their Risk in the media material -- and the reader will find concrete discussion and examples of news stories there.

On a more abstract level, do we want to be careful to use different words for different kinds of uncertainty? This of course leads to philosophical questions about the taxonomy of uncertainty, but let me quote one prescription for using three different families of words in three different contexts.

Where uncertainty is assessed qualitatively, it is characterised by providing a relative sense of the amount and quality of evidence (that is, information from theory, observations or models indicating whether a belief or proposition is true or valid) and the degree of agreement (that is, the level of concurrence in the literature on a particular finding). This approach is used ..... through a series of self-explanatory terms such as: high agreement, much evidence; high agreement, medium evidence; medium agreement, medium evidence; etc.

Where uncertainty is assessed more quantitatively using expert judgement of the correctness of underlying data, models or analyses, then the following scale of confidence levels is used to express the assessed chance of a finding being correct: very high confidence at least 9 out of 10; high confidence about 8 out of 10; medium confidence about 5 out of 10; low confidence about 2 out of 10; and very low confidence less than 1 out of 10.

Where uncertainty in specific outcomes is assessed using expert judgment and statistical analysis of a body of evidence (e.g. observations or model results), then the following likelihood ranges are used to express the assessed probability of occurrence: virtually certain >99%; extremely likely >95%; very likely >90%; likely >66%; more likely than not > 50%; about as likely as not 33% to 66%; unlikely <33%; very unlikely <10%; extremely unlikely <5%; exceptionally unlikely <1%.

[Quoted verbatim from page 5 of the IPCC Climate Change 2007: Synthesis Report. That material was extracted from a document Guidance Notes for Lead Authors of the IPCC Fourth Assessment Report on Addressing Uncertainties which I often describe as "the most useful 4 pages you will ever read about uncertainty".]

3. How do "ordinary people" actually use phrases like "unlucky" or "slim chance"?

We get some insight from our study of "unprompted" references to chance in blogs. (xxx other literature?)

4. Distinctions suggested by word usage.

xxx start from actual or hypothetical uses of words, try to discern what underlying distinction is being employed.

Lucky vs fortunate?

xxx discuss