ORIGINAL ARTICLE Communicating Relative Risk Changes with Baseline Risk: Presentation Format and Numeracy Matter Nicolai Bodemer, PhD, Bjo¨rn Meder, PhD, and Gerd Gigerenzer, PhD Background. Treatment benefits and harms are often and harms, whereas a percentage format often impeded communicated as relative risk reductions and increases, understanding. For example, many participants misinter- which are frequently misunderstood by doctors and patients. preted a relative risk reduction as referring to an absolute One suggestion for improving understanding of such risk risk reduction. Participants with higher numeracy generally information is to also communicate the baseline risk. We performed better than those with lower numeracy, but all investigated 1) whether the presentation format of the base- participants benefitted from a frequency format. Limitations line risk influences understanding of relative risk changes are that we used a hypothetical medical scenario and a non- and 2) the mediating role of people’s numeracy skills. representative sample. Conclusions. Presenting baseline risk Method. We presented laypeople (N =1234)withahypothet- in a frequency format improves understanding of relative ical scenario about a treatment that decreased (Experiments risk information, whereas a percentage format is likely to 1a, 2a) or increased (Experiments 1b, 2b) the risk of heart lead to misunderstandings. People’s numeracy skills play disease. Baseline risk was provided as a percentage or a fre- an important role in correctly understanding medical infor- quency. In a forced-choice paradigm, the participants’ task mation. Overall, communicating treatment benefits and was to judge the risk in the treatment group given the rela- harms in the form of relative risk changes remains problem- tive risk reduction (or increase) and the baseline risk. atic, even when the baseline risk is explicitly provided. Key Numeracy was assessed using the Lipkus 11-item scale. Re- words: relative risk; absolute risk; risk communication; sults. Communicating baseline risk in a frequency format numeracy; presentation format; baseline risk. (Med Decis facilitated correct understanding of a treatment’s benefits Making 2014;34:615–626) ransparent and intuitive communication of communicated as a risk reduction of 20% in mortal- Thealth information is a major challenge in ity due to breast cancer.1 But what does that num- ensuring informed consent and shared decision ber actually mean? The epidemiological data on making. For instance, the benefit of mammography which this benefit is based show that about 5 in screening for women aged 50 and older can be every 1000 women without screening die from breast cancer within 10 years, as opposed to 4 in 1000 with screening—a relative risk reduction of 20%. Another way of conveying the same informa- Received 25 December 2012 from Max Planck Institute for Human tion is to say that participating in screening reduces Development, Harding Center for Risk Literacy, Berlin, Germany (NB, GG); and Max Planck Institute for Human Development, Center for breast cancer mortality by 0.1% (1 in 1000), Adaptive Behavior and Cognition (ABC), Berlin, Germany (NB, BM, namely, from 0.5% (5 in 1000) to 0.4% (4 in GG). This research was supported by the joint program ‘‘Acciones Inte- 1000). As this example illustrates, different formats gradas Hispano-Alemanas’’ from the Deutscher Akademischer Aus- exist for expressing treatment benefits. A relative tauschdienst (DAAD) and the Ministerio de Ciencia y Tecnologı´a. BM was supported by Grant ME 3717/2 from the Deutsche Forschungsge- meinschaft (DFG) as part of the priority program ‘‘New Frameworks of Rationality’’ (SPP 1516). Revision accepted for publication 8 February Supplementary material for this article is available on the Medical 2014. Decision Making Web site at http://mdm.sagepub.com/supplemental. Ó The Author(s) 2014 Reprints and permission: Address correspondence to Nicolai Bodemer, Harding Center for Risk http://www.sagepub.com/journalsPermissions.nav Literacy, Max Planck Institute for Human Development, Lentzeallee DOI: 10.1177/0272989X14526305 94, 14195 Berlin, Germany; e-mail: [email protected]. MEDICAL DECISION MAKING/JULY 2014 615 Downloaded from mdm.sagepub.com at Max Planck Institut on June 13, 2014 BODEMER AND OTHERS Table 1 Relative and Absolute Risk Changes Type of Change Risk Reduction (RR) Risk Increase (RI) ER ER ER ER Relative (R) RRR 5 ControlÀ Treatment RRI 5 TreatmentÀ Control ERControl ERControl Example: Example: 0:5% 0:4% 0:028% 0:014% RRR 5 0:À5% 5 20% RRI 5 0:014À % 5 100% Absolute (A) ARR 5 ER ER ARI 5 ER ER Control À Treatment Treatment À Control Example: Example: ARR 5 0:5% 0:4% 5 0:1% ARI 5 0:028% 0:014% 5 0:014% À À Note: The risk reductions are based on the mammography example (breast cancer mortality reduction from 5 in 1000 to 4 in 1000 when participating in screening). The example for the risk increase measures is based on the ‘‘pill scare’’ (increase of thrombosis from 1 in 7000 to 2 in 7000 when taking the third-generation contraceptive pill). ERControl = event rate in the control group (baseline risk); ERTreatment = event rate in the treatment group. risk reduction (RRR) is defined as the difference Relative Risk Reduction and Baseline Risk between event rates (risk) in the control and treat- 16 ment groups, normalized by the event rate in the Schwartz and others found that providing the control group (the baseline risk). An absolute risk baseline risk improved accuracy in estimating breast reduction (ARR), by contrast, is defined as the dif- cancer mortality when presenting women with a treat- ference between the risk in the control group and ment’s benefit as an RRR or an ARR. Even when pro- the risk in the treatment group (Table 1). vided with the baseline risk, however, about two- The same holds true for communicating potential thirds of the participants still gave incorrect mortality harms of treatments. The relative risk increase (RRI) rate estimates. Similarly, most participants in Sheri- 17 and the absolute risk increase (ARI) are defined anal- dan and colleagues’ study were unable to calculate ogously to risk reductions (Table 1). For instance, in the effect on a given baseline risk when presented 1995 the UK Committee on Safety for Medicine with an RRR or an ARR; only about one-fifth of partic- used the RRI format in its statement that the third gen- ipants correctly estimated the effect in the treatment eration of the contraceptive pill increased the risk of group based on the given information. Natter and 15 life-threatening blood clots twofold, that is, by Berry showed that omitting the baseline risk led to 100%, compared with the second-generation pill.2,3 an overestimation of both the baseline risk and the This RRI corresponded to an ARI of 0.014% (increase risk for the treatment group, whereas providing the from 1 in 7000 to 2 in 7000). baseline risk led to more accurate estimates overall. Whereas relative and absolute risk formats are derived from the very same data (event rates in the The Role of Numeracy in Understanding Relative control and treatment groups), they are often not Risk Reduction With Baseline Risk psychologically equivalent. For instance, laypeople as well as health professionals evaluate treatment ben- Both Schwartz and Sheridan and their col- 16,17 efits more favorably when they are presented as an RRR leagues also investigated numeracy—the ability to 18 rather than an ARR.4–8 Nonetheless, RRR remains the comprehend and use numerical information —as dominant format for communicating treatment benefits amoderatingvariableinpeople’sunderstandingof in direct-to-consumer advertisements, in patient deci- RRR with baseline risk. They found that participants sion aids, and within the medical community.9–14 with lower numeracy particularly had difficulties solv- One suggestion for improving understanding of rela- ing the tasks. For instance, in one study, the proportion tive reductions (or increases) is to also communicate of correct answers ranged from 5.8% for those with the the baseline risk.15 lowest numeracy score to 40% for those with the high- 16 However, several important questions remain, est score. Similarly, in another study, only 5% of par- given that neither the role of the presentation ticipants with lower numeracy calculated the event rate format of the baseline risk nor the influence of in the treatment group correctly, whereas up to 50% of 17 people’s numeracy in interpreting relative risk those with higher numeracy did. These findings are changes with baseline risk have been systematically consistent with related research showing that people’s investigated. 616 MEDICAL DECISION MAKING/JULY 2014 Downloaded from mdm.sagepub.com at Max Planck Institut on June 13, 2014 BASELINE RISK AND RELATIVE RISK INFORMATION numeracy mediates their understanding of statistical of people at risk), thus helping to clarify the meaning information, risk perception, and decision making.18–25 of an RRR. Berry and others28 provided participants with risk The Role of Presentation Format in Communicating increase information, either in a relative or an abso- Baseline Risk lute format or as number needed to treat. When no baseline risk was provided, participants strongly What format should be used to communicate base- overestimated the side effect, particularly in the RRI line risk? Whereas previous studies often used fre- condition. Provision of baseline risk improved esti- quency formats, percentages are also frequently mates, and no differences between the 3 formats used. This holds both for scientific articles and were found. Yet even with baseline risk information, when reporting results to the public. Examples the risk increase was overestimated. Because, how- include statements like ‘‘In the United Kingdom, ever, participants received the baseline risk in both 35% of deaths are due to cardiovascular causes, com- a percentage and a frequency format, the study does pared with about 60% in those with type 2 diabetes not allow for assessing potential differences between and 67% of type 1 diabetic patients over 40 years different baseline risk formats.