Background: Informed medical decisions require understanding the benefits and risks of treatments. This entails comparing treatment outcomes to a control group. The incremental risk format has been recommended as it directly visualizes the differences between treatment and control group in one graph, whereas they have to be calculated from two separate graphs in the total risk format. We investigated when the incremental risk format aids understanding.
Methods: In two experiments, participants received information about medical treatments, either as incremental or total risk format. We assessed verbatim knowledge (precise quantitative knowledge), gist knowledge (knowledge of essential meaning), and evaluations of the formats. Study 1 (N = 99) consisted of only one trial with medical information and additionally assessed recall. Study 2 (N = 222) assessed learning across multiple trials and additionally varied the presence of feedback and the number of treatment options.
Results: In Study 1, the incremental risk format (vs. total risk format) led to worse knowledge, recall, and evaluations. In Study 2, participants learned to understand the incremental risk format over time, resulting in comparable verbatim knowledge and evaluations as the total risk format, and in even better gist knowledge. Feedback and number of treatment options did not moderate the effect of risk format.
Limitations: The studies were conducted with non-patient samples and Study 2 employed hypothetical treatments.
Conclusions: The incremental risk format was initially less understandable than the total risk format. After a short learning period, however, the incremental risk format resulted in better gist knowledge and was comparable otherwise, which suggests that participants had to get used to that format. This has important implications for the study of new formats.

This article is openly accessible at: https://doi.org/10.1177/0272989X20904357