6.8: Rates
It is helpful to know the absolute numbers of cases of a particular disease, especially to understand the need for healthcare resources. However, this number doesn’t tell us how much the disease or condition affects the population. The case numbers should also be assessed relative to the population size. Health officials need to know whether a disease or illness is affecting a large or small portion of the population that could be affected. This helps epidemiologists determine how big the problem is, and sometimes helps to discover how transmissible a disease is or how certain risk factors may be emerging. Is this disease or condition quite common or rare? Is it increasing in its effect over time or in proportion to the growth of the population? Are there other factors also high in this population that could have an association to the condition? (See Chapter 5 for more information on ecological studies that look at this question). For these reasons, prevalence and/or incidence are often expressed as a rate, rather than an absolute number. Rates can be percentages, or per 10,000 people, or 100,000 people depending on the size of the population. Using rates can also provide more manageable numbers with which to calculate statistics.
For example, perhaps we know that there are 38.4 million people living with diabetes in the United States. Since the total population in the U.S. in 2021 was 331.9 million, 11.6% of the population had diabetes ( National Diabetes Statistics Report , 2023). We could also express this as 384 cases per 100,000 people. Sometimes this format is easier to use when handling large numbers and trying to compare different populations. Now imagine that instead we were looking at the population of the UK which had 67.33 million people in 2021. Hypothetically, if the same absolute number of people in the UK had diabetes, that would make up 57% of their population instead of 11.6% - a much bigger problem! Instead, if we wanted to compare the effect of diabetes between the U.S. and the UK, we can use rates instead of absolute numbers. In reality, the UK estimates around 5 million people had diabetes in the year 2021 (JackW, 2023), which equates to 7.4% of their population, or 50 cases per 100,000 people. So not only does the UK have fewer people living with diabetes, they also have a lower rate of diabetes. See Table \(\PageIndex{1}\) for a tabulated comparison of these rates.
Table \(\PageIndex{1}\): Comparison of Diabetes Rates in the U.S. and UK 2021
|
Country |
Population in 2021 |
Number of people living with diabetes in 2021 |
Percentage |
Per 100,000 |
|---|---|---|---|---|
|
United States |
331.9 million |
38.4 million |
11.6% |
384 |
|
United Kingdom |
67.33 million |
5 million |
7.4% |
50 |
It’s also important to make sure we are calculating a case rate from the appropriate population - the population at risk for developing the disease. Not all diseases and conditions affect all populations, so we do need to make sure we are calculating the percentage of folks who have the disease out of the folks who could have the disease or condition. This is also called the attack rate - essentially how many people that disease or condition attacks out of the population at risk. Sometimes the entire population is at risk, or could potentially be diagnosed, as in our earlier example of diabetes. But other times only a portion of the population may be at risk - for example, workers in a particular industry exposed to asbestos or silica dust who may be at risk for developing lung disease from that exposure. It would not be accurate to count workers in another industry when trying to determine rates of lung damage for workers exposed to those environmental toxins. A great example of the use of attack rates for outbreak investigation is with foodborne illnesses. Epidemiologists can calculate attack rates and use that information to discover the potential source or sources of the illness.