| NICE, Cochrane, and the English NHS recommend talking about natural frequencies and not percentages. They also say that if you have to use percentages you should use absolute numbers, not relative numbers. Take something really simple about percentages. What does 0.1% mean? Only 1 in 4 people know this means "1 in 1000". https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3310025/ > Gigerenzer et al show how only 25% of the general population could correctly identify 1 in 1000 as being the equivalent of 0.1%. Take something a little bit more complex, such as the relative increase in risk vs the actual total risk. We know that most people do not understand what a 75% increase in risk means in real terms. But most people do understand a simpler explanation: of people who don't eat $THING we'd expect to see 1 person out of 1000 people over ten years developing a disease, but if 1000 people all eat $THING every day over ten years we'd expect to see about 2 people developing the disease. See also cancer screening: (This is a good useful link that rattles through most of what I'd want to say) https://www.nice.org.uk/guidance/ng34/evidence/expert-paper-... “If you participate in breast screening, you will reduce your chances of dying from breast cancer in the next 10 years by 24%” versus “If you participate in breast screening, you will reduce your chances of dying from breast cancer in the next 10 years from 37 in 10,000 to 28 in 10,000” BMJ has more about relative vs absolute risk: https://bestpractice.bmj.com/info/toolkit/practise-ebm/under... Here's a final example. For some time we knew members of the public couldn't do this, but we thought healthcare professionals could. Turns out that they couldn't do it either. Both groups find it much easier if you convert this into natural numbers and probability trees. "A machine has been invented to scan a population for a disease. The machine is good but not perfect. If you have the disease there is a 90% chance it will return positive. If you do not have the disease there is a 1% chance it will return positive. About 1% of the population have the disease. Mr Smith is tested, and the test comes back positive. What's the chance Mr Smith actually has the disease?" (This is from "Reckoning with Risk" by Gerd Gigerenzer). Most people cannot get the right answer from this question. If you reword the question they can. "Think of a group of 100 people. 1 of them has a disease. The entire group is screened. The one person who has the disease tests positive. Of the 99 people who don't have the disease one person will also test positive. How many people of those who test positive have the disease?" You can also show this as a probability tree. https://imgur.com/a/JWVQRxI Two books I recommend are "Reckoning With Risk" and "Risk Savvy", both by Gigerenzer. |