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The question was: Would you get a cancer screening done if it reduced your chance of dying from this type of cancer by one third?

If you were undecided, you may have wanted more information about the type of cancer. But there is a more important reason why you should not have decided based on the information you got in this scenario: the number (risk reduction by “one third”) is not meaningful at all, because it only gave you information about relative risk reduction, rather than absolute risk reduction. If you get this kind of information from your health care provider, you should ask: “Ok, and what are my chances of dying from this type of cancer without the treatment?” Or more generally, ask for your base risk: your risk without the treatment. Only with that information can you judge the benefit of a treatment. For example, if my initial risk of getting this type of cancer is 6%, the screening will reduce my risk to 4% (the absolute risk reduction would be 2% in that case). If my initial risk is 0.3%  (in other words, if only 3 in 1,000 people from my risk group are developing this cancer), the screening would reduce my risk to 0.2%, which means the absolute risk reduction in that case is only 0.1%. In other words, only 1 in 10,000 people would benefit from the screening in that case. In this second case, the screening would have a much smaller benefit, even if it also reduces my relative risk by one third. (Notice, however, how even the assumption of a base risk of 6%, which would be a very high base risk for any specific type of cancer, makes the treatment seem less urgent than it seemed in the original question.)

Here are different versions of how the information can be presented to you.

• Relative risk reduction: “If you have this test every 2 years, it will reduce your chance of dying from this cancer by around one third over the next 10 years.”
• Absolute risk reduction: “If you have this screening test every 2 years, it will reduce your chance of dying from this cancer from around 3 in 1,000 to around 2 in 1,000 over the next 10 years.”
• Number needed to treat: “If around 1,000 people have this screening test every 2 years, 1 person will be saved from dying from this cancer every 10 years.”

For all those questions, the benefits of the test are identical, except that they are expressed either as a relative risk reduction, as an absolute risk reduction, or as the number of people needed to be treated (screened) to prevent one death from cancer.

Unfortunately, studies overwhelmingly show that many patients do not understand this difference between relative and absolute risk reduction, and that they do indeed evaluate treatments much more favorably if the benefits are presented as relative risk reductions.

What is even more concerning is that doctors and other health professionals (including nurse educators, and even reviewers of grant proposals), succumb to the same bias. Multiple studies have shown that health professionals, just like patients, get confused by numbers about risk reduction, and rate the effectiveness of a treatment much higher when the benefits are described in terms of a relative risk reduction, rather than as an absolute risk reduction or a number needed to treat.

It is therefore quite clear that reporting relative risk reductions without clearly specifying the base rates is bad practice, because not only is the information so incomplete as to be meaningless, but it also leads people – patients as well as providers – to overestimate the benefits of treatments.

Yet unfortunately, this bad practice is still very common, as Gerd Gigerenzer and his colleagues report in an enlightening article. [Please get in touch if you would like to read it and don’t have access to the full text]. According to their review, even articles in leading medical journals often only report relative risk reduction. So do brochures and pamphlets that provide information for patients, although this is perhaps less surprising when the brochures are issued by those who benefit financially from providing treatments.

I’m with Gigerenzer when he advocates for better education and better practices in medical communication. In the meantime, the take home message is simple: when offered any risk reducing treatment, always find out as much as you can about your base risk before making a decision.

by Ursina Teuscher (PhD), at Teuscher Decision Coaching, Portland OR

Selected References:
Covey, J. (2007). A meta-analysis of the effects of presenting treatment benefits in different formats. Medical Decision Making, 27, 638–654.
Gigerenzer, G., Gaissmaier, W., Kurz-Milcke, E., Schwartz, L. M., & Woloshin, S. (2007). Helping Doctors and Patients Make Sense of Health Statistics. Psychological Science in the Public Interest, 8(2), 53–96.
Naylor, C.D., Chen, E., & Strauss, B. (1992). Measured enthusiasm: Does the method of reporting trial results alter perceptions of therapeutic effectiveness? Annals of Internal Medicine, 117, 916– 921.
Muhlhauser, I., Kasper, J., & Meyer, G. (2006). FEND: Understanding of diabetes prevention studies: Questionnaire survey of professionals in diabetes care. Diabetologia, 49, 1742–1746.
Sarfati, D., Howden-Chapman, P., Woodward, A., & Salmond, C. (1998). Does the frame affect the picture? A study into how attitudes to screening for cancer are affected by the way benefits are expressed. Journal of Medical Screening, 5, 137–140.