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Ann Intern Med. 2011 Jul 19;155(2):87-96. doi: 10.7326/0003-4819-155-2-201107190-00004.

Communicating data about the benefits and harms of treatment: a randomized trial.

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  • 1Veterans Affairs Outcomes Group, White River Junction, Vermont 05009, USA.



Despite limited evidence, it is often asserted that natural frequencies (for example, 2 in 1000) are the best way to communicate absolute risks.


To compare comprehension of treatment benefit and harm when absolute risks are presented as natural frequencies, percents, or both.


Parallel-group randomized trial with central allocation and masking of investigators to group assignment, conducted through an Internet survey in September 2009. ( registration number: NCT00950014)


National sample of U.S. adults randomly selected from a professional survey firm's research panel of about 30,000 households.


2944 adults aged 18 years or older (all with complete follow-up).


Tables presenting absolute risks in 1 of 5 numeric formats: natural frequency (x in 1000), variable frequency (x in 100, x in 1000, or x in 10,000, as needed to keep the numerator >1), percent, percent plus natural frequency, or percent plus variable frequency.


Comprehension as assessed by 18 questions (primary outcome) and judgment of treatment benefit and harm.


The average number of comprehension questions answered correctly was lowest in the variable frequency group and highest in the percent group (13.1 vs. 13.8; difference, 0.7 [95% CI, 0.3 to 1.1]). The proportion of participants who "passed" the comprehension test (≥13 correct answers) was lowest in the natural and variable frequency groups and highest in the percent group (68% vs. 73%; difference, 5 percentage points [CI, 0 to 10 percentage points]). The largest format effect was seen for the 2 questions about absolute differences: the proportion correct in the natural frequency versus percent groups was 43% versus 72% (P < 0.001) and 73% versus 87% (P < 0.001).


Even when data were presented in the percent format, one third of participants failed the comprehension test.


Natural frequencies are not the best format for communicating the absolute benefits and harms of treatment. The more succinct percent format resulted in better comprehension: Comprehension was slightly better overall and notably better for absolute differences.


Attorney General Consumer and Prescriber Education grant program, the Robert Wood Johnson Pioneer Program, and the National Cancer Institute.

[PubMed - indexed for MEDLINE]
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