Understanding Statistics Used to Estimate Risk and Recommend Screening

Approved by the Lineagotica Editorial Board, 08/2018

Statistics are estimates that describe trends in large numbers of people. Researchers use statistics to determine cancer risk and recommend screening.

Statistics can help estimate your risk of cancer. But they cannot tell you if you will develop cancer.

Ask your health care team for the most appropriate statistics for your situation. And ask them to explain the statistics that seem unclear.

This article describes the types of statistics used to estimate cancer risk.

Incidence

Incidence estimates how many people will be diagnosed with cancer. Researchers calculate this within a population during a certain time period. That time period is usually 1 year.

Here is how researchers calculate incidence of cancer for the current year:

  • They identify numbers of people diagnosed with cancer over a range of years.

  • They place those numbers within a statistical model to predict future incidence.

  • Researchers may use different ranges. These ranges depend on the type of statistical report.

For example, consider the American Cancer Society’s publication, Cancer Facts & Figures 2018. In it, researchers looked at the number of people diagnosed with cancer from 2000 to 2014. These numbers were used to calculate the estimated cancer incidence for 2018.

Research reports often give incidence as an incidence rate. This is the estimated number of people who will develop cancer per 100,000 people.

Example: The 2017 incidence rate for prostate cancer in the United States is 114.9. This means doctors diagnosed about 114.9 out of every 100,000 men in the country with prostate cancer in 2017.

Research reports often state incidence as an age-adjusted incidence rate. This accounts for age distribution differences when comparing populations. For example, one population might have more older adults than another.

Example: Florida has a large number of older adults. In comparison, most people who live in Alaska are young. And the incidence of breast cancer increases with age. This means that the absolute incidence is higher in Florida than in Alaska. But when researchers adjust for age, the numbers change. The annual age-adjusted incidence rate (cases per 100,000 women) for Alaska was higher than that of Florida from 2010 to 2014. The rate was 115.5 in Florida compared with 125.4 in Alaska.

Incidence statistics may cover large populations. For example, all people in the United States. Typically, large population statistics are calculated with information from a sample.

Statistics can also cover specific population groups. For instance, only black women. Researchers often call these types of statistics “specific.” For example, the specific incidence rate for breast cancer in black women is 125.5, per 100,000 women.

Incidence statistics may look at these factors:

  • Several cancers combined

  • Specific age ranges

  • Specific types of cancer

  • Specific stages of a type of cancer

  • Specific cancer risk factors. Risk factors increase a person’s chance of developing a certain type of cancer.

Prevalence

Prevalence estimates how many people have or have had cancer. Researchers calculate this estimate within a population at a certain time.

Incidence estimates the number of people newly diagnosed with cancer. In contrast, prevalence describes all people with cancer. The number includes these individuals:

  • Those newly diagnosed

  • Those receiving treatment

  • Those who had cancer treatment in the past

Researchers express prevalence as an absolute number or a percentage.

Prevalence rates show the number of people with cancer per 100,000 people.

Like incidence, researchers can also use prevalence to evaluate these factors:

  • Large populations

  • Specific population groups

  • Several cancers combined

  • Specific types of cancer

  • Specific stages of a type of cancer

  • Cancer risk factors

Example: A woman with genetic mutations in either her BRCA1 and BRCA2 genes has a higher risk of breast cancer. The prevalence of mutations of 1 of these 2 genes is less than 1%. This means that less than 1% of women have a mutated BRCA1 or BRCA2 gene. However, the prevalence of a BRCA gene mutation among women with breast cancer is around 5% to 10%. This means that out of all women who have breast cancer, 5% to 10% have a BRCA gene mutation.

Mortality

Mortality describes how many people die from cancer during a given time. A mortality rate is the number of deaths from cancer per 100,000 people during a time period. That time period is usually 1 year.

Researchers can calculate mortality rates for specific cancer types and population subsets.

Examples of population subsets:

  • Children younger than 12

  • People who smoke

  • Women with the BRCA1 gene mutation

Researchers can also give age-adjusted mortality rates.

Mortality rates change greatly with advances in treatment, screening, and prevention.

Example: In the early 1960s, the age-adjusted mortality rate for Hodgkin lymphoma in the United States was greater than 1.55. That means there were 1.55 deaths per 100,000 people. Then, doctors introduced combination chemotherapy in the late 1960s. As a result, the age-adjusted mortality rate dropped to 0.3, based on deaths between 2011 and 2015.

Estimating a person’s cancer risk to recommend screening

Researchers can estimate which groups of people may have an increased risk of developing certain types of cancer. They look at incidence and prevalence statistics for a cancer type in various groups.

For example, statistics tell us that:

  • Older women are at higher risk for breast cancer than younger women.

  • Black men are at higher risk for prostate cancer than white men.

  • People who drink alcohol often are at higher risk for liver cancer than people who do not drink alcohol.

  • Combining incidence and prevalence statistics with mortality statistics helps inform cancer screening recommendations.

Example:

  • Prevalence and incidence statistics show that colorectal cancer is one of the most common cancers in the United States.

  • Age-specific prevalence and incidence rates also show that colorectal cancer is most common in people over age 50.

  • The mortality rates for colorectal cancer show treatment is much more successful when doctors find it early.

Combing this information, doctors recommend that, routine screening for colorectal cancer start at age 50. This is the recommendation for people with an average risk. And the goal is to increase the likelihood of prevention or early detection.

Doctors also consider other risk factors when making individual screening recommendations:

  • Family history

  • Presence of other illnesses

  • Various lifestyle factors

Statistics adapted from the American Cancer Society's publication, Cancer Facts & Figures 2018, and the National Cancer Institute Surveillance Epidemiology and End Results (SEER) database.

Related Resources

Understanding Statistics Used to Guide Prognosis and Evaluate Treatment

Cancer Screening

Understanding Cancer Risk