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