# Understanding Cancer Statistics

Statistics are often used in news reports or in talking about someone's risk of getting
cancer or whether a treatment will work. These statistics can be confusing or misleading
if you don't understand how they are used. Statistics are collected and analyzed to
help people better understand what is being observed. There are many examples of how
statistics are used in our daily life. They include average temperature and median
house price. Statistics are also used to understand the probability or chance of something
happening—of winning the lottery or being struck by lightning. Statistics help people
make a *best guess* of any one case, but they can't guarantee that something will or won't happen.

Statistics are often used in cancer to help guide decision-making, identify people at risk of getting cancer, and figure out the best test or treatment. But they can't be used to know for certain what will happen to any one person. Below is an explanation of some of the more commonly used statistics:

### Mean (average)

A mean (average) is when all the numbers are added up and divided by the number of people or whatever is being measured. For example, let's take the average age of a group of people. In the group is a person who is 20 years old and another person who is 60 years old. The total of their ages is 80. If you divide that by 2 (the number of people), then the average age would be 40. But neither person is 40. Similarly, the average age at retirement or death can be calculated, but it does not mean that any one person actually retires or dies at that age.

### Median

The median is the halfway point when counting a group of numbers. Half the numbers are below and half are above the median. For example, housing prices may range from $50,000 to $350,000. If 5 houses are $50,000, $60,000, $85,000, $350,000, and $350,000 each, the median price is $85,000. The average or mean price is $179,000.

## Cancer facts and figures

Every year, the American Cancer Society publishes *Cancer Facts & Figures,* a booklet that lists the number of people expected to get cancer during that year,
how long someone may survive, and the number of people expected to die from cancer
that year. This information is provided by type of cancer, state, gender, age, and
other information. Other cancer-related behaviors are also published. These include
the number of people smoking or getting Pap smears or mammograms. This information
is calculated using formulas and statistical models based on collected data already
collected. These statistics describe large groups of people. They do not take into
account a person's individual risk factors, such as family history, behaviors, or
early detection practices. Below are some statistics covered in *Cancer Facts & Figures.*

### Lifetime risk

The lifetime risk is one person's chance of getting or dying of cancer over a lifetime. That risk changes based on the person's age.

### Relative risk

The relative risk compares the risk of people getting a cancer with certain risk factors (family history or certain behaviors like smoking) with a similar group of people without those risk factors. It is usually referred to as X-times or X-fold relative risk when compared with the other group of people.

### Incidence rates

The incidence rate is the number of people who get a particular cancer for every 100,000 people. This allows comparisons across different groups of people by state, age, or some other factor. This is different from the actual number of people getting cancer. For example, it was estimated that 559 men and 460 women per 100,000 people in Massachusetts had cancer between 2007 and 2011. This statistic compares with 499 men and 396 women in California. The actual number of people who were diagnosed with cancer between 2007 and 2011 may be different.

### Relative survival rate

The relative survival rate is the portion of people surviving with cancer after adjusting for normal events occurring that affect life expectancy. These include things such as accidents or dying of other diseases. The people included in this statistic reflect how many people with cancer are alive after a certain time (usually 5 years). They may still have cancer or be free of their cancer. An individual's prognosis may be different from this statistic based on many factors, such as general health, type and stage of the cancer, and response to treatment.

### Mortality rates

The mortality rate (or death rate) is the number of people who die from a particular cancer for every 100,000 people. This allows comparisons across different groups of people by state or some other factor. This is different from the actual number of people dying of cancer. For example, 211 men and 149 women per 100,000 people in Massachusetts and 187 men and 137 women per 100,000 in California were estimated to have died of cancer between 2007 and 2011. The actual number of people who died in this time period may be different.

### Prognosis

People with cancer may want to know their prognosis. The prognosis is the likely outcome or course of a person's cancer. It is the chance of recovery or recurrence and of dying from their disease. Like other statistics, this information is a prediction of the chance of things occurring and is based on a variety of factors. These factors include type and stage of the cancer, type and response to treatment, and other personal factors such as general health. All of this information can help people make decisions about changing behaviors, taking tests or treatments, and overall outlook. But one should always remember that this information can never guarantee that something will or won't happen.

** Medical Reviewers: **

- LoCicero, Richard, MD
- Sather, Rita, RN