Medical Decision Making (MDM)
July–August 2012; 32 (4)
Theme: Patients’ Choices: Perceived Risk, Health State Values, and Decisions
Original Articles/Presenting Probabilities to Patients
The Influence of Graphic Display Format on the Interpretations of Quantitative Risk Information among Adults with Lower Education and Literacy: A Randomized Experimental Study
Kirsten J. McCaffery, Ann Dixon, Andrew Hayen, Jesse Jansen, Sian Smith, and Judy M. Simpson
Med Decis Making July–August 2012 32: 532-544, first published on November 10, 2011 doi:10.1177/0272989X11424926
Objective To test optimal graphic risk communication formats for presenting small probabilities using graphics with a denominator of 1000 to adults with lower education and literacy.
Methods A randomized experimental study, which took place in adult basic education classes in Sydney, Australia. The participants were 120 adults with lower education and literacy. An experimental computer-based manipulation compared 1) pictographs in 2 forms, shaded “blocks” and unshaded “dots”; and 2) bar charts across different orientations (horizontal/vertical) and numerator size (small <100, medium 100–499, large 500–999). Accuracy (size of error) and ease of processing (reaction time) were assessed on a gist task (estimating the larger chance of survival) and a verbatim task (estimating the size of difference). Preferences for different graph types were also assessed.
Results Accuracy on the gist task was very high across all conditions (>95%) and not tested further. For the verbatim task, optimal graph type depended on the numerator size. For small numerators, pictographs resulted in fewer errors than bar charts (blocks: odds ratio [OR] = 0.047, 95% confidence interval [CI] = 0.023–0.098; dots: OR = 0.049, 95% CI = 0.024–0.099). For medium and large numerators, bar charts were more accurate (e.g., medium dots: OR = 4.29, 95% CI = 2.9–6.35). Pictographs were generally processed faster for small numerators (e.g., blocks: 14.9 seconds v. bars: 16.2 seconds) and bar charts for medium or large numerators (e.g., large blocks: 41.6 seconds v. 26.7 seconds). Vertical formats were processed slightly faster than horizontal graphs with no difference in accuracy. Most participants preferred bar charts (64%); however, there was no relationship with performance.
Conclusions For adults with low education and literacy, pictographs are likely to be the best format to use when displaying small numerators (<100/1000) and bar charts for larger numerators (>100/1000).