Understanding Disease Frequency: Incidence, Prevalence, and Public Health Measures
Definition of Proportion
- A measure that states a count relative to the size of the group.
- A ratio in which the numerator is part of the denominator.
- May be expressed as a percentage.
Definition of Rate
- A ratio that consists of a numerator and a denominator and in which time forms part of the denominator.
- Contains the following elements:
- Disease frequency
- Unit size of population
- Time period during which an event occurs
Example of Rate Calculation
Definition of Prevalence
The number of existing cases of a disease or health condition in a population at some designated time.
Interpretation of Prevalence
Provides an indication of the extent of a health problem.
Example 1: Prevalence of diarrhea in a children’s camp on July 13 was 15.
Example 2: Prevalence of obesity among women aged 55-69 years was 367 per 1,000.
Uses of Prevalence
- Describing the burden of a health problem in a population.
- Estimating the frequency of an exposure.
- Determining allocation of health resources such as facilities and personnel.
Point Prevalence
Period Prevalence
Definition of Incidence
The number of new cases of a disease that occur in a group during a certain time period.
Incidence Rate (Cumulative Incidence)
Describes the rate of development of a disease in a group over a certain time period.
Contains three elements:
- Numerator = the number of new cases
- Denominator = the population at risk
- Time = the period during which the cases occur
Incidence Rate Calculation (IWHS Data)
Attack Rate (AR)
- Alternative form of incidence rate.
- Used for diseases observed in a population for a short time period.
- Not a true rate because time dimension is often uncertain.
- Example: Salmonella gastroenteritis outbreak
- Formula:
Incidence Density
An incidence measure used when members of a population or study group are under observation for different lengths of time.
Formula for Incidence Density
Interrelationship Between Prevalence and Incidence
- If duration of disease is short and incidence is high, prevalence becomes similar to incidence.
- Short duration–cases recover rapidly or are fatal.
- Example: Common cold
- If duration of disease is long and incidence is low, prevalence increases greatly relative to incidence.
- Example: HIV/AIDS prevalence
Crude Rates, Measures of Natality
- Crude birth rate
- Fertility rate
- General
- Total
- Infant mortality rate
- Fetal death rate
- Neonatal mortality rate
- Postneonatal mortality rate
- Perinatal mortality rate
- Maternal mortality rate
Crude Birth Rate
General Fertility Rate
Total Fertility Rate
This rate is “the average number of children that would be born if all women lived to the end of their childbearing years and bore children according to a given set of age-specific fertility rates.”
In the United States, the total fertility rate was 2.06 in 2012. This rate is close to the replacement fertility rate of 2.1.
Fetal Death Rate
Used to estimate the risk of death of the fetus associated with the stages of gestation.
