Purpose: To determine how people who differ in numeracy and age perceive risks and report intentions to use a prescribed medication when presented with numeric and/or non-numeric information about the likelihood of side effects.
Methods: An internet sample of 1,527 participants was given side-effect information in one of six formats (the list format used in the US, verbal labels recommended in Europe, percentage, frequency, verbal labels plus percentage, verbal labels plus frequency). They responded to two risk perception questions, one likelihood-to-take-the-drug question on a 7-point scale, and stated the main reason for their likelihood rating.
Results: Given non-numeric risk information, 70-80% of participants overestimated risks compared to12-37% in numeric conditions. Non-numeric participants also reported being less likely to take the medication than numeric participants. Those in the US-list condition, in particular, were more likely to state that the rare severe side effect was the main reason for their likelihood rating than other participants who were more likely to state that side effects were neither likely nor severe. Of import, differences between numeric and non-numeric formats were greater for the highly numerate, but were also shown by the less numerate–an unexpected finding based on prior speculation. Age differences existed, with less numerate older adults not showing the same numeric advantage. Providing verbal labels (common, rare) with numeric information attenuated numeracy differences and reduced risk overestimation compared to all other conditions.
Conclusions: The US-list format for presenting side effects led to the greatest risk overestimation and focus on severe side effects relative to all other tested formats. The provision of risk in numeric formats compared to non-numeric ones had similar effects across numeracy levels. However, the effect of providing numbers may be more problematic for older less numerate adults, perhaps due to lower comprehension of numbers or number meaning or increased anxiety in the presence of unfamiliar numbers. Overall, this study revealed that providing numeric plus verbal likelihoods for side effects in decision aids and patient medication information is likely to generate more accurate risk perceptions across numeracy and age groups, which in turn may lead to better health outcomes. More research is needed to better understand how less numerate older populations react to the provision of numeric information.