What is a risk?

A risk is the chance that something will happen. For example, if you smoke a packet of cigarettes a day for 30 years, research suggests you have a 10 percent risk of dying from lung cancer.[1]

When treating a patient, doctors use numbers from research studies to tell them which treatments are likely to work for that person. These numbers also tell them about the risk of side effects. If you're a man and your surgeon says you need your prostate removed, there's a risk you'll have erection problems afterwards. Your surgeon may think the risk is too low to worry about. But you may think any chance is too high. This is why you need to understand what risk means – so you can take part in treatment decisions.

Imagine you're tossing a coin. It has two sides: heads and tails. So, you have a 1 in 2 chance that it'll come up heads, and a 1 in 2 chance that it'll come up tails. It's the same chance every time, however many times you flip it.

A 1 in 2 chance can also be written as a 50 percent chance. 'Percent' just means 'out of a hundred', so 50 percent looks like this:

Here's a medical example. Imagine your doctor says, "There is a 50 percent chance you will be cured by this drug." If 100 people like you were treated, the chances are that 50 of them (the red dots above) would not be cured, while 50 (the white dots) would recover. Here are two more examples:

The thing to remember is that, in both cases, the white dots show your chance of being fine.

If you see numbers like 0.8 percent, this means the risk is less than 1 in 100. The more zeros there are after the decimal point, the lower the chances. For example:

  • 0.008 percent risk is 8 in 100,000
  • 0.0008 percent risk is 8 in 1 million.

Those are the basics of the chance that something will happen to you. Don't worry if it seems difficult. Everyone has trouble with it. Just bear in mind that a low chance of something happening does not mean that there is no chance. Remember that 1 person out of 100 (one of the dots in our diagrams), still means one person will have that side effect.

In the next section, we'll explain ways that you can use chance or risk to understand the effects of treatments.

How do I use risk to choose treatments?

Risk can be useful for seeing how well a treatment works.

Let's say we thought that aspirin stopped you from getting a heart attack. To see if this was true, we would do a study. In this study, some people would take aspirin and others would not. Then we would see if people who took aspirin had fewer heart attacks than those who didn't take it. The study would run for five years.

Now let's say that these were the results of the study:

  • If you didn't take aspirin, your risk of having a heart attack was 2 percent over five years.
  • If you did take aspirin, your risk of having a heart attack was 1 percent over five years.

This is what this looks like:

So the study would say that aspirin reduced your chance of heart attack by 1 percent (down from 2 percent). This is called absolute risk reduction.

But we could also say that aspirin cut your chances of a heart attack in half, from 2 percent to 1 percent. And half is the same as 50 percent. So we could say that aspirin reduces your chances by 50 percent, which is called relative risk reduction.

Both the absolute and relative values are telling you about the same reduction in risk. Both are describing the same effect of aspirin.

If you heard only that aspirin reduced your risk of heart attack by 50 percent, you would probably be very interested. But how interested would you be to hear that aspirin reduced your risk of heart attack by 1 percent?

Ask your doctor if the numbers he or she gives you are the absolute or relative risk.

You may wish to ask your doctor to work out risks based on this.

Getting away from the numbers

If you are not a 'numbers person', there are other ways to think about the risks that you may find easier to understand.

  • Graphs: These show in pictures what the numbers mean.
  • Comparing the risk to other risks: For example, you might say that each year the increased risk of dying from smoking one cigarette is very small. It is similar to the risk of being struck by lightning, twice.
  • Descriptions: These are words for talking about risk like 'high' or 'low'. They are much easier for people to understand, but they are not very precise. People may have very different ideas about what high or low means.[2]

Showing risk in pictures

One of the advantages of graphs is that they can show the change in both absolute risk and relative risk in one picture.[2]

Let's imagine a new study, about heart attacks. Let's say your surgeon told you that an operation on the arteries of your heart would reduce your risk of dying from a heart attack from 20 percent to 10 percent.

This is what it looks like as a graph.

The graph gives you a good sense of what the risk was and how it changed. But it can also show another piece of useful information. Without surgery, there was a 20 percent chance of having a heart attack, but an 80 percent chance of not having a heart attack at all.

How do I compare risks?

To understand risks that are smaller than 1 percent (or 1 in 100) you may find it helpful to compare these risks to other risks in life.

You may have seen news reports about women who are on the contraceptive pill having an increased risk of blood clots in their legs.

These reports scared many young women away from taking the pill.

But the risk of a blood clot for a young woman taking the birth-control pill is about the same as the risk of a person being struck by lightning. This risk is a little more than 1 in 10,000.[3]

Here's another example: If you have a flu jab, there's a 1 in 1 million risks of getting Guillain-Barré syndrome, a rare disease that can make you paralyzed.[4] That is about the same as the risk of dying in an accident while on a 10-mile bicycle ride.

In the chart below, you will see the risk of some other rare things that happen.[1] You may want to print out this chart and ask your doctor how the risks associated with the treatments of your illness compare.

Some common risks The chance they will happen
Dying in a road traffic accident over 50 years of driving 1 in 85
Needing emergency treatment in the next year from injury by a can, glass bottle, or jar 1 in 1000
Needing emergency treatment in the next year from injury by a bed, mattress, or pillow 1 in 2000
Dying in any accident at home in the next year 1 in 7100
Being hit in your home by a crashing airplane 1 in 250,000
Drowning in the bath in the next year 1 in 685,000


How do I talk about risk?

Some people use words like 'high' or 'low' to talk about risk. If you want your doctor to do most of the thinking about risk, you can ask for a description in words like this. But your doctor may have a different idea of what these words mean than you do.

So an expert in risk communication has produced a scale that looks at particular risks and suggests words that doctors can use to describe them.[3] Here is an outline of the scale.

Risk description Percentage Fraction
High 1 More than 1 in 100
Moderate 0.1 1 in 100 to 1 in 1000
Low 0.01 1 in 1000 to 1 in 10,000
Very Low 0.001 1 in 10,000 to 1 in 100,000
Minimal 0.0001 1 in 100,000 to 1 in 1,000,000
Negligible 0.00001 Less than 1 in 1,000,000


Summing up the numbers

After reading this introduction to understanding risk, you should be better prepared to weigh up your options when you make decisions about your health. You can ask your doctor to explain the risks and benefits of any treatment he or she recommends, and work with your doctor to make decisions based on this information.

If you prefer graphs to numbers or the other way around, ask your doctor if it is possible to have risk shown to you in a way you understand.

Reviewed by: Glyn Elwyn

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  1. Paling J. Up to your armpits in alligators? How to sort out what risks are worth worrying about! Risk Communication and Environmental Institute, Gainesville, U.S.A.; 1997.
  2. Edwards A, Elwyn G, Mulley A. Explaining risks: turning numerical data into meaningful pictures. BMJ. 2002; 324: 827-830.
  3. Mohanna K, Chambers R. Risk matters in healthcare: communicating, explaining and managing risk. Radcliffe Medical Press, Abingdon, UK; 2001.
  4. Bennett P, Calman K (editors). Risk communication and public health. Oxford University Press, Oxford, UK; 2000.
  5. Hayden M, Pignone M, Phillips C, et al. Aspirin for the primary prevention of cardiovascular events: summary of the evidence. Annals of Internal Medicine. 2002; 136: 161-172.