Understanding Cancer Statistics
Statistics are often used in news reports or in talking about someone's risk for getting
cancer or whether a treatment will work. These statistics can be confusing or misleading
if you do not 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. This includes average temperature and median
house price. In addition, statistics are 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 situation, but they cannot guarantee that something will or won't happen.
Statistics are often used in cancer to help guide decision-making, identify people
at risk for getting cancer, and identify 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). An average, or mean, 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 and if you divide that by two (the
number of people), then the average age would be 40 and yet 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 of cancer that
year. This information is provided by type of cancer, state, gender, age, etc. Other
cancer-related behaviors, such as the number of people smoking or getting Pap smears
or mammograms, are also published. This information is calculated using formulas and
statistical models based on previously collected data. 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 percentage of people surviving with cancer after
adjusting for normal events occurring that affect life expectancy such as accidents,
dying of other diseases, etc. 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 of 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.
People with cancer may want to know their prognosis. The prognosis is the likely outcome
or course of a person's cancer—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