Sometimes, particularly for extremely rare conditions, we only need to know how many people are sick. How many cases of disease X or health behavior Y were there?
From surveillance data see chapter 3 , we know that the expected number of cases of meningococcal meningitis in a given year is zero. Therefore, 6 cases constitute a level quite above what is expected and would be termed an epidemic see chapter 3. For rare conditions like this one, simply knowing how many cases there are is sufficient for a proper public health response.
Since we normally expect none, officials at OSU and the local health department just needed to know that there were 6 students with meningococcal meningitis in order to mount a response in this case, requiring students 25 years old and younger to be vaccinated.
The above picture is useful from a public health infrastructure perspective [1] ; if you work at the health department in Josephine County, then you might want to keep a few doses of rabies vaccine on hand since cases of rabies in animals are often discovered because the infected animal bit a human, who must then be vaccinated. However, if you work at the health department in Wallowa County, which has had no recorded cases of animal rabies in 10 years, then maybe your resources would be better spent on things other than vaccine doses that will likely expire before they are used assuming you could quickly get doses of the vaccine from the state or neighboring counties if they ever became necessary.
Counts are less useful if we want to compare 2 populations. For instance, 1, cases of flu in Ashland, New Hampshire, versus , cases of flu in New York City—we cannot compare these 2 figures at a glance, because the denominators i. There are 2 commonly used measures of disease frequency that incorporate denominator information: one is a measure of existing disease prevalence , and the other is a measure of new disease incidence.
Incidence is used to study causes of disease, whereas prevalence is used more for resource allocation. Prevalence is a proportion, meaning that everyone who appears in the numerator must also appear in the denominator. This also means that prevalence ranges from zero no one has the disease to one everyone has the disease , and it is usually expressed as a percent.
Prevalence gives us a snapshot of the population-level disease burden at a given time. The formula for prevalence is. Looking at the formula for prevalence, you can see that everyone in the numerator is also in the denominator. Like counts, prevalence is used for resource-planning purposes. Consider the following question a public health authority might be faced with: How much money should our county health department spend on health education about smoking versus on physical activity?
One metric for deciding might be which behavior is more prevalent in the local population. The numerator for prevalence is all current cases. Thus prevalence is affected both by the rate at which new cases occur the incidence , see below and by how long people typically live with disease. Prevalence is therefore less useful for conditions such as a cold or the flu where people recover quickly because once they recover, they are no longer a prevalent case.
In such instances, we sometimes calculate period prevalence instead, which is just prevalence of disease over the course of a longer time frame: for example, what was the prevalence of flu in Newport, OR during the entire flu season? The numerator here would be all of the cases that occurred at any time during those months counting only the first instance if anyone was unlucky enough to have influenza twice , and the denominator would be everyone who lived in Newport during those same months.
Note that prevalence does not have units though providing the specified time is often appropriate and never wrong. If there are 5, students who live in the dorms at Oregon State University OSU , and during winter term , of them had the flu at some point, then the prevalence of flu was.
The above is an example of a period prevalence , since we were calculating it over a time period longer than one day. It is also an example of the specified time being calendar time—for everyone involved, the specified time was the winter term. In , 48, babies were born in Oregon. At 14 weeks postpartum, 33, of them were being breastfed, and had died. What is the prevalence of breastfeeding at 14 weeks postpartum? Here we need to subtract the infants who died before 14 weeks from the denominator, as they are no longer part of the population:.
This is not a period prevalence, because everyone was assessed on one day—we have just spread those days out throughout the year.
You can also reverse the calculations to establish the number of people with a disease, given the prevalence and population size. In a report on bone health by the Centers for Disease Control and Prevention CDC , v the authors reported that the prevalence of osteoporosis among men aged 65 and older was 5. According to data from the US Census Bureau, vi as of July 1, , there were an estimated 22,, men and 28,, women aged 65 and older living in the US.
Applying the prevalence, we can estimate that:. Incidence is a tricky word in epidemiology, because while it is always a measure of new cases, there are 2 possible denominators and at least a half-dozen words that all refer to this same thing. The numerator for incidence is always the number of new cases of a disease observed over some time period.
This means that, to study incidence, you must 1 follow people for some length of time the length varies according to the disease—a few hours or days for a foodborne illness versus a few decades for some cancers and 2 start with a population at risk —that is, people who are at risk of developing the disease at risk of becoming a case.
Usually, at a minimum, we therefore exclude people who already have the disease—such people cannot become an incident case because they are already a prevalent case. We also exclude anyone not capable of getting the disease, either because they are immune or because they lack the proper organs e.
Furthermore, because you are establishing the number of new cases, it is always necessary to include time-based units when reporting an incidence. One way of calculating incidence is to include in the denominator the number of people who were at risk of getting the condition at the start of your follow-up time period.
This calculation yields the incidence proportion. The incidence proportion is interpreted as the average risk chance of developing the disease over some time period. If there are 25 students in a particular class, and one person came to class on Monday of the first week already sick with the flu this person is a prevalent case—they are already sick, so are not at risk , and 2 more people got the flu on Wednesday of that same week, then what was the incidence of flu during Week 1?
Our numerator would be the number of new cases of flu—here, 2. Utilization of hospital services Measures of disease frequency are used to describe how common an illness or other health event is with reference to the size of the population the population at risk and a measure of time.
There are two main measures of disease frequency: 1. Prevalence Prevalence measures the proportion of individuals in a defined population that have a disease or other health outcomes of interest at a specified point in time point prevalence or during a specified period of time period prevalence. Example Of 10, female residents in town A on 1st January , 1, have hypertension. Incidence In contrast to prevalence, incidence is a measure of the number of new cases of a disease or other health outcome of interest that develops in a population at risk during a specified time period.
There are two main measures of incidence: Risk or cumulative incidence Is related to the population at risk at the beginning of the study period Rate Is related to a more precise measure of the population at risk during the study period and is measured in person-time units.
Risk Risk is the proportion of individuals in a population initially free of disease who develop the disease within a specified time interval. For example, some participants may: Develop the outcome under investigation Refuse to continue to participate in the study Migrate Die Enter the study some time after it starts To account for these variations during follow up, a more precise measure can be calculated, the incidence rate.
Incidence Rate Incidence rates also measure the frequency of new cases of disease in a population. Calculation of person-time at risk The denominator in an incidence rate is the sum of each individual's time at risk and is commonly expressed in person years at risk. Figure 1: Person-time at risk Person-time at risk is a measure of the number of persons at risk during the given time-period.
Issues in defining the population at risk For any measure of disease frequency, precise definition of the denominator is essential for accuracy and clarity [1] The population at risk denominator should include all persons at risk of developing the outcome under investigation.
Therefore, individuals who currently have the disease under study or who are immune e. However, this is not always possible in practice [1] Note that when individuals not at risk of the disease are included in the denominator population at risk the resultant measure of disease frequency will underestimate the true incidence of disease in the population under investigation 5.
The relationship between prevalence and incidence The proportion of the population that has a disease at a point in time prevalence and the rate of occurrence of new disease during a period of time incidence are closely related [1]. Prevalence depends on: 1. The incidence rate r 2. The duration of disease T For example, if the incidence of a disease is low but the duration of disease i. Other commonly used measures of disease frequency in epidemiology Measures of effect Measures of effect are used in epidemiological studies to assess the strength of an association between a putative risk factor and the subsequent occurrence of disease.
These measures are often collectively referred to as measures of relative risk The relative risk is a measure of the strength of an association between an exposure and disease and can be used to assess whether a valid observed association is likely to be causal [1]. The most commonly used measure of effect is the ratio of incidence rates that is: Rate or risk in exposed group Rate or risk in unexposed group 7.
Interpreting measures of relative risk RR A relative risk of 1. A relative risk Note: Rate ratios and risk ratios tend to be numerically similar for rare diseases. Turn on Animations. Field Epidemiology Manual Wiki A set of online resources for professionals working in intervention epidemiology, public health microbiology and infection control and hospital hygiene. FEM Wiki.
It looks like your browser does not have JavaScript enabled. Please turn on JavaScript and try again. Assessing the burden of disease and risk assessment Descriptive data analysis Analysis by person characteristics Analysis by place characteristics Choosing an appropriate type of map Which indicator to map?
Point prevalence survey of healthcare associated infections and antimicrobial use in European acute care hospitals? Page Content. Learning Objectives: After reading this chapter, you will be better able to: appreciate the definition of risk, rate, prevalence and odds; appreciate the difference between measures of disease occurrence and measures of effect; familiarize with the different terms and synonyms which are used to describe risk, rates, prevalence; understand that the epidemiological jargon is not always correct an attack rate is actually a risk ; identify what is the best measure to calculate in different study designs.
This denominator represents the total time the population was at risk of and being watched for disease. Thus, the incidence rate is the ratio of the number of cases to the total time the population is at risk of disease.
In a long-term follow-up study of morbidity, each study participant may be followed or observed for several years. One person followed for 5 years without developing disease is said to contribute 5 person-years of follow-up. What about a person followed for one year before being lost to follow-up at year 2? Therefore, the person followed for one year before being lost to follow-up contributes 1. The same assumption is made for participants diagnosed with the disease at the year 2 examination — some may have developed illness in month 1, and others in months 2 through So, on average, they developed illness halfway through the year.
The denominator of the person-time rate is the sum of all of the person-years for each study participant. So, someone lost to follow-up in year 3, and someone diagnosed with the disease in year 3, each contributes 2. Example A: Investigators enrolled 2, women in a study and followed them annually for four years to determine the incidence rate of heart disease. After one year, none had a new diagnosis of heart disease, but had been lost to follow-up.
After two years, one had a new diagnosis of heart disease, and another 99 had been lost to follow-up. After three years, another seven had new diagnoses of heart disease, and had been lost to follow-up.
After four years, another 8 had new diagnoses with heart disease, and more had been lost to follow-up. The study results could also be described as follows: No heart disease was diagnosed at the first year.
Heart disease was diagnosed in one woman at the second year, in seven women at the third year, and in eight women at the fourth year of follow-up. One hundred women were lost to follow-up by the first year, another 99 were lost to follow-up after two years, another were lost to follow-up after three years, and another women were lost to follow-up after 4 years, leaving women who were followed for four years and remained disease free.
Calculate the incidence rate of heart disease among this cohort. The incidence proportion underestimates the true rate because it ignores persons lost to follow-up, and assumes that they remained disease-free for all four years.
Example B: The diabetes follow-up study included diabetic women and 3, nondiabetic women. By the end of the study, 72 of the diabetic women and of the nondiabetic women had died. The diabetic women were observed for a total of 1, person years; the nondiabetic women were observed for a total of 36, person years.
Calculate the incidence rates of death for the diabetic and non-diabetic women. Prevalence, sometimes referred to as prevalence rate , is the proportion of persons in a population who have a particular disease or attribute at a specified point in time or over a specified period of time.
Prevalence differs from incidence in that prevalence includes all cases, both new and preexisting, in the population at the specified time, whereas incidence is limited to new cases only. Point prevalence refers to the prevalence measured at a particular point in time. It is the proportion of persons with a particular disease or attribute on a particular date.
Period prevalence refers to prevalence measured over an interval of time.
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