Zach's Facts: How do you know if COPD is worse in one state than another?
Learn about crude and age-adjusted rates and why they are important when measuring the burden of disease.
Editor's Note: Zach is the Lung Association data and statistics guru. In addition to monitoring trends in lung disease, analyzing data for major reports and running the Lung Association's Epidemiology and Statistics Unit, he makes sure we understand what the facts actually mean.
COPD is the third leading cause of death in the U.S. We know it impacts some states more than others, but comparing one state to another isn't exactly apples to apples. Understanding who is most impacted by disease is important in pinpointing who has the greatest need of support and resources, and is essential to effective public health interventions.
So when measuring the burden of a disease like COPD, you have to factor in the age and size of a state's population to get an accurate comparison. If you look at charts that illustrate COPD data, you might notice they say "age-adjusted" or, in the case of the CDC map below, "age-standardized" death rate.
So what exactly does age-adjusted death rate mean? How does it help us compare? Let's take a look at Florida and Texas to see how it all works.
Death Rate (aka Crude Rate)
An easy place to start our comparison is with the number of deaths. In 2014, there were more COPD deaths in Florida than in Texas. Based on COPD deaths, Florida is 16 percent higher than Texas.
However, these counts could be misleading if one state has a much larger population than the other. If one state has more people, then you would expect more deaths. Dividing the number of deaths by the total population corrects for this by giving a death rate (called a crude rate), a kind of per capita metric for comparing the likelihood of dying from COPD in each state. Since Florida (20 million) actually has a smaller population than Texas (27 million), death rates suggest Florida has a 57 percent higher death rate than Texas.
But looking just at the death rate is not an entirely fair comparison, as the likelihood of dying from COPD depends on a characteristic for which the two states greatly differ: the ages of the people who live there. COPD, and especially COPD deaths, affect older groups much more than younger ones. In fact, 97 percent of COPD deaths are in people age 55 and older, but the portion of the population in this age range is very different for each state. Florida is the popular place to retire, and with 32 percent of its population 55 and older, it is a much "older" state than Texas at only 22 percent.
Because COPD deaths are so much more common among people 55 and older, the overall crude death rate for COPD is skewed upward for Florida and downward for Texas.
Age-Adjusted Death Rates
Fortunately, there is a method that adjusts for differences in age profiles, allowing for more accurate comparisons. Age-adjusted death rates are obtained by calculating the death rate for each age group in each state, then applying these rates to a standard population (which is an established baseline used for measurement, such as the U.S. population in 2000.) It requires some math, but this method allows for a comparison where each state has the same portion of the population ages 55 and older, and thus, is more accurate.
Our example above used two age groups for simplicity (younger or older than 55) but using more provides greater accuracy. The table below shows the details of the age adjustment process using six age groups.
After adjusting for differences in age, the COPD death rates for the two states are very similar, with Texas (39.6) now higher than Florida (37.2) by a small margin.
Since most diseases vary by age, adjusting for age is a standard practice. However, many other factors can be adjusted for in the same way, such as race, sex or education level. These adjustments lead to a more accurate understanding of who is most burdened by any particular disease, and where we can best offer support and resources and make the greatest impact in communities and regions nationwide. Here at the Lung Association, this is critical to our work of improving the quality of life for those living with lung diseases like COPD, asthma, lung cancer and more.