By Miriam Kuppermann, PhD, MPH and Robert Nease, PhD

Last updated November 8, 2000

When researchers talk about a person's risk of developing a disease, they use a very specific language and a standard way of displaying this information. It is much easier to understand your own risk, or that of your family, if you also understand the language that scientists use.

What is Risk?

When talking about risk, there are three things that affect how risk is portrayed: the different possible outcomes, the chance of the outcomes happening, and the time periods being considered.

1. The outcome is the end result of what you are discussing in terms of risk. For example, the outcome may be that a person develops cancer or that they do not. The worse the outcome would be for you, the riskier you are likely to feel your situation is. For example, having a ten percent chance of developing cancer probably feels like a much higher risk than having a ten percent chance of developing a common cold.

2. Chance involves how likely the outcome is to occur. For example, when discussing cancer risk, you may discuss the chance of developing cancer versus the chance of not developing cancer.

3. The time period is how far in the future the risk level extends. Does it define your risk of getting the disease this year, or your risk of getting the disease in your lifetime? The longer you wait for an event, the more likely it is to happen. For example, you might have a low chance of developing cancer this year, but you have a much higher chance of developing cancer during your lifetime. So when your risk is presented over a long time period, it might seem higher to you than the same risk presented over a shorter time period.

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What Are Some Ways of Talking About Chance?

People describe chance in many different ways. One way is with numbers, such as: "There is a five percent chance that I will develop cancer in my lifetime," or "1 out of 20 people like me will develop cancer in their lifetime." Although those statements describe the same risk level, they may sound like a higher or lower risk to some people. In many cases, people use graphs to describe risk. These can often provide a clear visual impression of risk level that is easier to understand than numbers. There are several different kinds of graphs, each of which has benefits in some situations.

Numbers

Using numbers to talk about chance is very straightforward - you are simply told your numeric chances of the event. Often, people talk about numbers in terms of some number of people out of 100 or out of 1000.

• Percentages. We often use percentages to describe the chances of something happening. "Percent" is really shorthand for "how many in 100." So, a 50 percent chance of something happening (like a heads coming up on a coin toss) means that if you tossed that coin 100 times, on average, a heads would come up 50 times. Often percentages are written using the % sign (for example, 50 percent = 50%). Describing risk in terms of percent works well for risk over one percent (or 1 in 100). However, below one percent you have to describe risk as a fraction of a percent. For example, if an event occurs only 0.2 times out of 100, then that would be a 0.2 percent risk. This chance is too low to be easily described by percentages.
• A number in 1,000. For risks below 1 percent, it is sometimes easier to think in terms of some number in 1,000. In the above example where an event takes place .2 times out of one hundred, that risk can be described as 2 per 1,000. This approach is often an easier way of describing small numbers.

Pictures

There are many different ways to depict risk using pictures or charts. For the following charts, we will show the data for the number of people in a given family of 20 who prefer carrots (5), peas (7), or broccoli (8).

• Vertical bar chart. A fairly common way of presenting chance is called a vertical bar chart. In this kind of graph, the bars represent one possible outcome or group of people - in this case each bar represents one type of vegetable that a family member could like. The height of the bar represents the number of people.

• Pie chart. The pie chart is another common method for presenting chances. The pie chart shows what fraction of the total falls into each category. In our example, if 5 family members prefer carrots, that is 25 percent; 7 family members (or 35 percent) prefer peas; and 8 family members (or 40 percent) prefer broccoli.

A lot of people like pie charts, but recent research has shown that people can have a difficult time understanding and dealing with risk information presented in the pie chart format. Using pie charts is also more difficult with small chances. If a person has a one percent chance of developing disease A, and two percent chance of developing disease B, and a 97 percent chance of being disease-free, a pie chart representing this risk would look like this:

Even though the risk for disease B is double that of disease A, it's hard to tell the difference between small risks with the pie chart.


• Wall of balls. This presentation method is more unusual, but people have an easier time understanding risk with this depiction. If we go back to the example of the family of 20 people we can clearly see 5 people who prefer carrots, 7 who prefer peas, and 8 who prefer broccoli:

This method is particularly useful for small chances. For example, if we go back to the chances of developing disease A (1 percent) or disease B (2 percent) the wall of balls looks like this:

• The same small chance would look like this with the vertical bar chart or the pie chart (as we saw above):

• Recent studies suggest that using a wall of balls helps people more quickly understand and compare chances.
  
  

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What's Important to Remember When Thinking About Risk?

Once you understand what these numbers all mean and what your risks are, there are a few other issues you need to keep in mind:

Perspective

A medication appears less attractive when it's described as having a 5 percent chance of side effects than when it's described as having a 95 percent chance of producing no side effects

We've all heard of the difference between thinking a glass is half full or half empty. Both are true, but how you feel about a situation depends in part on your perspective; that is, how you look at it. The same is true when you're thinking about chances.

When we talk about perspective, we mean that how you think about a risk depends on whether you focus on the chance of something bad happening as opposed to the chance of it not happening. Suppose, for example, that for every 100 people treated with a medication, 5 had an unpleasant side effect, and 95 did not. Studies have shown that the perspective makes a difference in how people feel about such risks. For example, such a medication appears less attractive when it's described as having a 5 percent chance of side effects than when it's described as having a 95 percent chance of producing no side effects. Because both are true, it's important to keep both chances in mind when thinking about risk.

Absolute Risk vs. Relative Risk

The way people think about risk can change depending on whether those risks are presented in absolute terms or relative terms. Suppose, for example, that you have a 2 percent chance of getting a specific disease during the next year. Now suppose that there's a medicine that will reduce your risk to 1 percent. That is, the medicine reduces your chance of getting the disease from 2 percent to 1 percent.

There are two ways to think about how effective the medicine is. The absolute reduction in your risk is 1 percent (i.e., 2 percent without the medicine minus 1 percent with the medicine). The other way is to view the medicine as cutting your chances in half, (i.e., from 2 percent to 1 percent). This second way of thinking about things is called relative risk reduction.

People may be swayed more than they should be by risks that are presented as relative risks than absolute risks

Studies suggest that people may be swayed more than they should be by risks that are presented as relative risks rather than absolute risks. In the example, cutting a risk in half sounds more impressive than reducing a risk by 1 percent. Furthermore, someone reducing their risk from 10 percent to 9 percent gets the same benefit as someone reducing their risk from 2 percent to 1 percent (i.e., a 1 percent reduction in risk), even though the first person only gets a 10 percent relative reduction (1 percent out of 10 percent) and the second gets a 50 percent relative reduction (1 percent out of 2 percent).

We will present both absolute and relative risks. In general, keep in mind the absolute changes - both good and bad - in risks that can happen from screening, testing, and treatment.

Risk vs. Diagnosis vs. Death

Remember that having a risk factor for a disease isn't the same as having the disease, and that having the disease doesn't mean that you'll die from it. For many conditions, only a portion of people with risk factors actually get the disease. Of those who do, only a portion die from it. And for many conditions, survival from the time of diagnosis can be substantial. For example, suppose you have a 10 percent risk for developing a disease, and if you do develop the disease you have a 20 percent chance of dying from the disease. Graphically, these numbers look like this:

Competing risks may change over time

It is important to remember that the risk factors you might have for certain diseases will change over time. As a result, you may feel differently about your chance of developing a disease at different times in your life. If someone learns that they have a 5 percent chance of developing breast cancer, for example, their concern about this risk might diminish with age because it pales in comparison with their increased risk for heart disease. It also might feel different if they learned about this risk at age 70 instead of age 40, and if they have other serious medical conditions

Each person has individual risk factors, and these risks often change over time. You need to keep in mind that, even if this risk stays the same, how you feel about it today may be different than how you'll feel in the future.

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Chance Landmarks

Some of the risks discussed in this web site may be easy to understand. For example, if the chance of something happening is 50 percent, that means that out of every 2 people, 1 person will have that event happen. Other risks are less easy to understand. For example, if you have a risk of 3 out of 1,000 people, how likely is that to happen? Is it a high risk, or a low risk?

To help put these risks in perspective, we have compiled some "chance landmarks" to keep in mind as you think about risks. These show the chances of common events happening in your life as a comparison point.

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