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--average temperature, median house price, etc. 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 about identifying people at risk for getting cancer and identifying 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 understanding 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 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 five 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.
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. For example, the lifetime risk of a woman in the US getting breast cancer is a 1 in 8 chance if she lives to be 85. But her immediate risk will change over her lifetime and will be lower when she is younger and higher when she is older. Her risk of getting breast cancer before the age of 40 is only about 1 in 208, while her risk after the age of 70 is about 1 in 16.
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. For example, men who smoke have about 23 times higher risk of developing lung cancer than nonsmoking men.
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 604 men and 456 women per 100,000 people in Massachusetts had cancer between 2001 and 2005. This statistic compares with 518 for men and 396 for women in California. The actual number of people who were diagnosed with cancer between 2001 and 2005 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 five 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, response to treatment, etc.
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, 238 men and 166 women per 100,000 people in Massachusetts and 206 men and 151 women per 100,000 in California were estimated to have died of cancer between 2001 and 2005. 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 happen.
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