David Budescu, Ph.D., the Anne Anastasi Chair in Psychometrics and Quantitative Psychology, teamed up with researchers at the University of Illinois-Champaign Urbana to measure how well the public understands phrases such as virtually certain, very likely, likely, unlikelyand very unlikely when reading the findings of climate change research.

These terms are among seven simple qualifiers used by the Intergovernmental Panel on Climate Change (IPCC) to describe scientists’ research findings to policymakers and the public.

In the context of the reports, each term is meant to correspond to a range of numerical probabilities (i.e.,virtually certain refers to 99 percent or higher probability,very likely refers to 90 percent or higher probability, andlikely means 66 percent or higher probability).

But Budescu’s research revealed that the public usually perceived the verbal terms as meaning something less extreme than the scientists had intended.

The study asked 223 volunteers to weigh in on their interpretation of a group of 13 IPCC sentences containing the various phrases. For example, one sentence read: “It is very likely that hot extremes, heat waves and heavy precipitation events will continue to become more frequent.” Less than 5 percent of the volunteers interpreted the probability of that statement consistent with IPCC guidelines, the study showed.

For example, in another sentence, “Average Northern Hemisphere temperatures during the second half of the 20th century were very likely higher than during any other 50-year period in the last 500 years,” a quarter of all volunteers interpreted the phrase “very likely” to mean a probability lower than 70 percent rather than 90 percent.

“The paradoxical . . . consequences of the ‘one size fits all’ solution adopted by the IPCC are that the report may lead to underestimation of the magnitude of the problems being discussed,” Budescu and colleagues wrote.

However, supplementing the verbal phrases with numbers considerably improved communication, the report said. The researchers recommended that the IPCC use both words and numbers to communicate uncertainty and probability.

The study, which was funded by the National Science Foundation, appeared in the March issue ofPsychological Science.