Have you ever heard a speaker say something like this? “One in 5 of us will die of heart disease. There are 20 people in this room, so 4 of you will die of heart disease.”
This statement illustrates a misconception about risk. I’m going to talk about heart disease but the concept is true for any risk assessment–breast cancer, diabetes, car wreck, bicycle wreck. (I’m also not 100% sure of this statistic. I have seen it in a couple slides but a quick search only turned up “1 in 5 will have heart disease” and having a disease is a far cry from dying of it.)
Suppose you gave everyone a deck of cards. The deck only has 5 cards: an ace of hearts, and 4 clubs. Everyone draws one card. Each person’s risk is 20%. It doesn’t matter what card the guy next to you drew. You still have a 20% risk of drawing the ace of hearts.
Our fictional speaker believes that there is only one deck, and 20% of the deck is aces of hearts. If the guy next to you drew the last ace of hearts, your risk is 0%. Or if you are drawing the last card, and all but one of the aces of hearts have been drawn, and all the clubs have been drawn, your risk is 100%.
When we think about individual risk, everyone has been given their own deck of cards. But we don’t all get the same deck. Age, inactivity, obesity, smoking, family history, gender, geography, and ethnicity are risk factors for heart disease that stack the deck. I’ve dealt myself more clubs by bicycling for transportation. Each year I age, I get more aces of hearts in my deck.
There is at least one ace of hearts in everyone’s deck. Any piece of equipment can malfunction, we all have one and only one heart. Some of us have 3000 clubs or 3 million clubs, but even so we can still draw that one ace of hearts. Some of us have 9 aces of hearts and only 1 club, and manage to draw that one club.
So a better educated seminar speaker, first of all, wouldn’t claim that 1 in 5 of us will die of heart disease when it’s actually that 1 in 5 of us will merely have heart disease, and second, wouldn’t say that “4 of you will have heart disease” to a room of 20 people. Our speaker could more accurately say to an audience of 20 million that “4 million of you, give or take a couple standard deviations, will have heart disease”.
Any speaker worth her salt (or salt substitute) would go on to describe the lifestyle changes we can make to stack the deck in our favor.
Go ride a bicycle. You can put the cards in your spokes.