This video explores the concepts of absolute and relative risk, illustrating how they can be misunderstood and misused in decision-making. It uses examples like heart attack and cancer risks to demonstrate the importance of understanding these differences when evaluating health-related choices.

Pause the video now if you want to figure it out for yourself! One common stumbling block is the difference between relative risks like these and what are called absolute risks. Risk is the likelihood that an event will occur. It can be expressed as either a percentage—for example, that heart attacks occur in 11% of men between the ages of 60 and 79—or as a rate—that one in two million divers along Australia's western coast will suffer a fatal shark bite each year. These numbers express the absolute risk of heart attacks and shark attacks in these groups. Changes in risk can be expressed in relative or absolute terms. For example, a review in 2009 found that mammography screenings reduced the number of breast cancer deaths from 5 women in 1,000 to 4. The absolute risk reduction was about 0.1%, but the relative risk reduction from 5 cases of cancer mortality to 4 is 20%. Based on reports of this higher number, people overestimated the impact of screening. To see why the difference between the two ways of expressing risk matters, let's consider the hypothetical example of a drug that reduces heart attack risk by 40%. Imagine that out of a group of 1,000 people who didn't take the new drug, 10 would have heart attacks. The absolute risk is 10 out of 1,000, or 1%. If a similar group of 1,000 people did take the drug, the number of heart attacks would be 6. In other words, the drug could prevent 4 out of 10 heart attacks—a relative risk reduction of 40%. Meanwhile, the absolute risk only dropped from 1% to 0.6%, but the 40% relative risk decrease sounds a lot more significant. Surely preventing even a handful of heart attacks, or any other negative outcome, is worthwhile, isn't it? Not necessarily. The problem is that choices that reduce some risks can put you in the path of others. Suppose the heart attack drug caused cancer in one half of 1% of patients. In our group of 1,000 people, 4 heart attacks would be prevented by taking the drug, but there would be 5 new cases of cancer. The relative reduction in heart attack risk sounds substantial, and the absolute risk of cancer sounds small, but they work out to about the same number of cases. In real life, everyone's individual evaluation of risk will vary depending on their personal circumstances. If you know you have a family history of heart disease, you might be more strongly motivated to take a medication that would lower your heart attack risk, even knowing it provided only a small reduction in absolute risk. Sometimes we have to decide between exposing ourselves to risks that aren't directly comparable.