DDP - Measuring Disease Occurrence Lesson

Measuring Disease Occurrence

Frequency: Ratios

Common frequency measures are ratios, proportions, and rates. A ratio is the relative magnitude of two quantities or a comparison of any two values.

Ratios are common descriptive measures, used in all fields. In epidemiology, ratios are used as both descriptive measures and as analytic tools. As a descriptive measure, ratios can describe the male-to-female ratio of participants in a study, or the ratio of controls to cases. For analysis, ratios can be calculated for occurrence of illness, injury, or death between two groups. A ratio that is used frequently in epidemiology is the death-to-case ratio.

Death-to-case ratio is the number of deaths attributed to a particular disease during a specified period divided by the number of new cases of that disease identified during the same period. It is used as a measure of the severity of illness: the death-to-case ratio for rabies is close to 1, that is, almost everyone who develops rabies dies from it, whereas the death-to-case ratio for the common cold is close to 0.

Proportions

A proportion is the comparison of a part to the whole. It is a type of ratio in which the numerator is included in the denominator. You might use a proportion to describe what fraction of students have chickenpox, or what percentage of the student population is less than 18 years of age. A proportion may be expressed as a decimal, a fraction, or a percentage. In epidemiology, proportions are used most often as descriptive measures. For example, one could calculate the proportion of children in a school vaccinated against measles, or the proportion of students who developed illness on a school field trip. Proportions are also used to describe the amount of disease that can be attributed to a particular exposure. For example, on the basis of studies of smoking and lung cancer, public health officials have estimated that greater than 90% of the lung cancer cases that occur are attributable to cigarette smoking.

Prevalence is the proportion of persons in a population who have a particular disease or attribute at a specified interval of time. The numerator for prevalence includes all persons during a specified interval of time that have the particular disease or attribute. The denominator for prevalence includes all persons during a specified interval or point in time. For example, a hearing examination survey of 4277 persons between the ages of 52 and 85 years showed that 510 had hearing problems. The prevalence of the condition was

$\frac{510}{4277}=0.119$, then $0.119\ast100=11.9\%$

Incidence is the proportion of persons in a population who develop a disease or demonstrate an attribute during a specified period of time. The numerator for incidence includes only those persons who develop the condition during the specified time period or new cases. The denominator for incidence includes all persons exposed to risk of developing the disease or condition during the specified time and therefore, measures risk. For example, in a study of 1680 women between 16 and 35 years of age, it was found that 315 used oral contraceptives. Twenty-seven of the oral contraceptive users developed blood clots. The incidence was therefore:

$\frac{27}{315}=0.086$, then $0.086\ast100=8.6\%$

Incidence and prevalence are defined as fractions and are usually presented in decimal form. Incidence and Prevalence are often expressed as cases per 100 or cases per 1,000. In the examples above, the Incidence proportion is 11.9 cases per 100 people and the Prevalence proportion is 8.6 cases per 100 people.

Rates

In epidemiology, a rate is a measure of the frequency with which an event occurs in a defined population over a specified period of time. Because rates put disease frequency in the perspective of the size of the population, rates are useful for comparing disease frequency in different locations, at different times, or among different groups of persons with potentially different sized populations; that is, a rate is a measure of risk. A rate can also describe how quickly disease occurs in a population. For example, 70 new cases of breast cancer occur per 1,000 women per year. An attack rate is the proportion of the population that develops illness during an outbreak. For example, 20 of 130 students developed diarrhea after attending a school picnic. A case-fatality rate is the proportion of persons with the disease who die from it. It is a measure of the severity of the condition. For example, 5 deaths due to meningitis among County A's population of 13,200 with 1,500 meningitis cases results in a case-fatality rate of 3.3 cases per 1,000 people.

A mortality rate is a measure of the frequency of deaths in a defined population during a specified time period. Morbidity is a measure of the frequency of illness related to a disease or condition. Morbidity and mortality measures are often the same mathematically; it's just a matter of what you choose to measure, illness or death. The crude mortality rate is the mortality rate from all causes of death for a population.

Cause-Specific Example

In the United States in 2009, a total of 2,437,163 deaths occurred. The estimated population was 307,006,550. The crude mortality rate in 2009 was, therefore,

$\frac{2,437,163}{307,006,550}=0.007938$, $0.007938\ast100,000=793.8$ deaths per 100,000 population

The cause-specific mortality rate is the mortality rate from a specified cause for a population. The numerator is the number of deaths attributed to a specific cause. The denominator is the size of the population at the midpoint of the time period. The fraction is usually expressed per 100,000 population.

In 2009, a total of 599,413 deaths were attributed to heart disease, yielding a cause-specific mortality rate of 195.2 per 100,000.

Infant Mortality Rate Example

The infant mortality rate is a commonly used measure for comparing health status among nations because it reflects the health of the mother and the child during pregnancy and one year after. The infant mortality rate is generally calculated on an annual basis. Health of the mother and infant, in turn, reflects a wide variety of factors, including access to prenatal care, prevalence of prenatal maternal health behaviors such as alcohol or tobacco use, postnatal care and behaviors, sanitation, and infection control.

 The birth rate is the total number of live births in a population over a period of time.

 

Calculate the Rates Activity

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