# Cancer Screening Overview (PDQ®): Screening - Patient Information [NCI]

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## How is Cancer Risk Measured?

Cancer risk is measured in different ways. The findings from surveys and studies about cancer risk are studied and the results are explained in different ways. Some of the ways risk is explained include absolute risk, relative risk, and odds ratios.

• Absolute risk

This is the risk a person has of developing a disease, in a given population (for example, the entire U.S. population) over a certain period of time. Researchers estimate the absolute risk by studying a large number of people that are part of a certain population (for example, women in a given age group). Researchers count the number of people in the group who get a certain disease over a certain period of time. For example, a group of 100,000 women between the ages of 20 and 29 are observed for one year, and 4 of them get breast cancer during that time. This means that the one-year absolute risk of breast cancer for a woman in this age group is 4 in 100,000, or 4 chances in 100,000.

• Relative risk

This is often used in research studies to find out whether a trait or a factor can be linked to the risk of a disease. Researchers compare two groups of people who are a lot alike. However, the people in one of the groups must have the trait or factor being studied (they have been "exposed"). The people in the other group do not have it (they have not been exposed). To figure out relative risk, the percentage of people in the exposed group who have the disease is divided by the percentage of people in the unexposed group who have the disease.

Relative risks can be:

• Larger than 1: The trait or factor is linked to an increase in risk.
• Equal to 1: The trait or factor is not linked to risk.
• Less than 1: The trait or factor is linked to a decrease in risk.

Relative risks are also called risk ratios.

• Odds ratio

In some types of studies, researchers don't have enough information to figure out relative risks. They use something called an odds ratio instead. An odds ratio can be an estimate of relative risk.

One type of study that uses an odds ratio instead of relative risk is called a case-control study. In a case-control study, two groups of people are compared. However, the individuals in each group are chosen based on whether or not they have a certain disease. Researchers look at the odds that the people in each group were exposed to something (a trait or factor) that might have caused the disease. Odds describes the number of times the trait or factor was present or happened, divided by the number of times it wasn't present or didn't happen. To get an odds ratio, the odds for one group are divided by the odds for the other group.

Odds ratios can be:

• Larger than 1: The trait or factor is linked to an increase in risk.
• Equal to 1: The trait or factor is not linked to risk.
• Less than 1: The trait or factor is linked to a decrease in risk.

Looking at traits and exposures in people with and without cancer can help find possible risk factors. Knowing who is at an increased risk for certain types of cancer can help doctors decide when and how often they should be screened.

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WebMD Public Information from the National Cancer Institute

Last Updated: February 25, 2014
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.
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