Epidemiology and Health Condition Rates

Epidemiology: The Science of Prevention

Defining Epidemiology

Epidemiology is the study of health-related trends in populations. The purposes of epidemiology are disease prevention, health maintenance, and health protection.

An epidemic is defined as an outbreak that occurs when there is an increased incidence of a disease beyond what would be normally found in the population.

Rates are the primary measurement used to describe either the occurrence or the existence of a specific state of health or illness.

Development of Epidemiology as a Science

Key figures in the development of epidemiology include:

John Graunt
William Farr
John Snow
Florence Nightingale

Nurses use epidemiological principles to provide health interventions to targeted groups. Epidemiological calculations provide numerical information about the impact of disease and death on populations and aggregates. The epidemiological process is a systematic method of targeting a specific health need with the goal of improving health.

Community health nurses are uniquely positioned to identify cases and recognize patterns of disease, eliminate barriers to disease control, and provide education and counseling targeted at a disease condition or specific risk factors.

Epidemiologic Models

Epidemiologic Triad
The Wheel of Causation
The Web of Causation
Natural History of Disease

Applying Epidemiologic Principles in Practice

Epidemiologic principles are applied in practice through:

Assessment of health needs and assets conducted at both individual and community levels.
Using assessment data for planning and implementing interventions.
Promoting healthy lifestyles.
Preventing and controlling outbreaks.
Contributing to a safe and healthy environment.
Evaluating the effectiveness of health services.

Describing Health Conditions: Understanding and Using Rates

Key Concepts

Concept and calculation of rates.
Crude, specific, and adjusted rates.
Incidence and prevalence rates.
Sensitivity and specificity calculations.

Epidemiologic descriptive studies have determined measurable risk factors for major illnesses. Epidemiology relies on statistical evidence to determine the rate of spread of disease and the proportion of people affected and is also used to evaluate the effectiveness of disease prevention and health promotion activities and to determine the extent to which goals are met.

Rate

A rate is the primary measurement used to describe the occurrence (quantity) of a state of health in a specific group of people in a given time period. A rate typically has four components:

A specified period of time.
The numerator (the # of people in whom an event occurred during a given time period).
The denominator (the total # of people at risk for the same period of time).
A constant.

Indices of the health of a community, region, or country include comparisons of general mortality rates and maternal-infant rates. specific rates calculated by person, place, and time provide the best description of a health condition.

Calculation of rates provides the best indicators of the probability that a specific state of health will occur.

Crude and Adjusted Rates

Adjusted rate: A statistical procedure that removes the effects of differences in the composition of a population, such as age, when comparing one population with another.
Crude rate: Measurement of the occurrence of the health problem or condition being investigated in the entire population.

Incidence and Prevalence Rates

Incidence and prevalence rates are used to measure the existence of a particular disease and allow comparison of the rate of disease in one population to another, even if the population numbers differ.

Incidence Formula: $(\frac{Number \, of \, new \, cases \, in \, the \, population \, at \, a \, specific \, time}{population \, total} \times 1,000)$
Prevalence Formula: $(\frac{Number \, of \, existing \, cases \, in \, the \, population \, at \, a \, specific \, time}{population \, total} \times 1,000)$

Types of Incidence Rates

Incidence density: Use of a person-time denominator in the calculation of rates.
Incidence rate: Measure of the probability that people without a certain condition will develop that condition over a period of time.
Relative risk ratio: The ratio of the incidence rate in the exposed group and the incidence rate in the nonexposed group.

Mortality Rates

Mortality rates provide information about the cause of death. Public health workers can examine overall death rates (crude mortality rate), deaths from specific causes (cause‑specific rate, case fatality rate), or deaths at specific times across the lifespan (infant mortality ratio, age‑specific rate).

Crude Mortality Rate Formula: $(\frac{Number \, of \, deaths}{population \, total} \times 1,000)$
Infant Mortality Rate Formula: $(\frac{Number \, of \, infant \, deaths \, before \, 1 \, year \, of \, age \, in \, a \, year}{number \, of \, live \, births \, in \, the \, same \, year} \times 1,000)$

Attack Rates

Endemic: A disease or condition is endemic when there is a moderate, ongoing occurrence in a given location.
Epidemic: Condition occurs when the rate of disease exceeds the usual (endemic) level of the condition in a defined population.
Pandemic: Condition occurs when an epidemic occurs in multiple countries or continents.
Attack Rate Formula: $(\frac{Number \, of \, people \, exposed \, to \, a \, specific \, agent \, who \, develop \, the \, disease}{total \, number \, of \, people \, exposed})$

Sensitivity and Specificity

Sensitivity: The ability of the test to correctly identify people who have the health problem under study. It is the probability of testing positive if the health problem is truly present.
Specificity: The ability of the test to correctly identify people who do not have the health problem. It is the probability of testing negative if the health problem is truly absent.

Prevalence is influenced by the number of people who have developed the condition in the past and how long the illness lasts. Prevalence rates provide important data for implementing prevention measures.