Expressing medicine side effects: Assessing the effectiveness of absolute risk, relative risk, and number needed to harm, and the provision of baseline risk information Dianne C. Berry a, * , Peter Knapp b , Theo Raynor b a Pro-Vice-Chancellor’s Office, University of Reading, Whiteknights House, Whiteknights, Reading RG6 6AH, UK b Pharmacy Practice and Medicines Management Group, University of Leeds, UK Received 3 April 2005; received in revised form 7 September 2005; accepted 8 September 2005 Abstract Objective: To assess the effectiveness of absolute risk, relative risk, and number needed to harm formats for medicine side effects, with and without the provision of baseline risk information. Methods: A two factor, risk increase format (relative, absolute and NNH) Â baseline (present/absent) between participants design was used. A sample of 268 women was given a scenario about increase in side effect risk with third generation oral contraceptives, and were required to answer written questions to assess their understanding, satisfaction, and likelihood of continuing to take the drug. Results: Provision of baseline information significantly improved risk estimates and increased satisfaction, although the estimates were still considerably higher than the actual risk. No differences between presentation formats were observed when baseline information was presented. Without baseline information, absolute risk led to the most accurate performance. Conclusion: The findings support the importance of informing people about baseline level of risk when describing risk increases. In contrast, they offer no support for using number needed to harm. Practice implications: Health professionals should provide baseline risk information when presenting information about risk increases or decreases. More research is needed before numbers needed to harm (or treat) should be given to members of the general populations. # 2005 Elsevier Ireland Ltd. All rights reserved. Keywords: Absolute risk; Relative risk; Number needed to harm; Baseline risk information 1. Introduction Over the past two decades there has been increasing recognition that patients both want and need to be given information about their illnesses and treatments, in a form that they can readily understand and apply to their own circumstances (e.g. [1,2]). The provision of such informa- tion enables patients to become active partners in decisions about their healthcare [3]. However, it is not the case that presentation of any information, in any format, will necessarily bring about beneficial effects. One type of information that has been noted to cause particular difficulties in terms of patient (and health professional) understanding is the presentation of risk information (e.g. [4]). It is now widely accepted that particular presentation formats can have significant effects on the way in which risk information is interpreted, and can influence intended and actual health behaviours. The two most commonly used methods for conveying comparative risk information are absolute and relative risk reductions (or increases). Absolute risk reduction can be thought of as the difference between risk of an event in a control group and risk of an event in a treatment group. In contrast, relative risk reduction is the ratio of risks of the treatment and control group. Thus, a risk reduction from 6 to 3%, for example, can be expressed as an absolute risk reduction of 3%, or as a relative risk reduction of 50%. A less commonly used method is the ‘number needed to treat’ (NNT), which is simply the reciprocal of www.elsevier.com/locate/pateducou Patient Education and Counseling 63 (2006) 89–96 * Corresponding author. Tel.: +44 118 378 7113; fax: +44 118 378 7424. E-mail address: D.C.Berry@Reading.ac.uk (D.C. Berry). 0738-3991/$ – see front matter # 2005 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.pec.2005.09.003