Rate=NumeratorDenominator=Number of health events in a specified periodPopulation in same area in same specified period×k When raw counts are converted to rates, the community health nurse can make meaningful comparisons with rates from other cities, counties, districts, or states; from the nation; and from previous time periods. These analyses assist the nurse in determining the magnitude of a public health problem in a given area and allow more reliable tracking of trends in the community over time (Box 5-3). BOX 5-3 Using Rates in Everyday Community Health Nursing Practice The following school situation exemplifies the value of rates: A community health nurse screened 500 students for TB in Southside School and identified 15 students with newly positive tuberculin tests. The proportion of Southside School students affected was 15/500, or 0.03 (3%), or a rate of 30/1000 students at risk for TB. Concurrently, the nurse conducted screening in Northside School and again identified 15 positive tuberculin tests. However, this school was much larger than the Southside School and had 900 potentially at-risk students. To place the number of affected students in perspective relative to the size of the Northside School, the nurse calculated a proportion of 15/900, or 0.017 (1.7%), or a rate of 17/1000 students at risk. On the basis of this comparison, the nurse concluded that although both schools had equal numbers of tuberculin conversions, Southside School had the greater rate of tuberculin test conversions. To determine if rates are excessively high, the nurse should compare rates with the city, county, and state rates and then explore reasons for the difference in these rates. TB, Tuberculosis. Sometimes a ratio is used to express a relationship between two variables. A ratio is obtained by dividing one quantity by another, and the numerator is not necessarily part of the denominator. For example, a ratio could contrast the number of male births to that of female births. Proportions can describe characteristics of a population. A proportion is often a percentage, and it represents the numerator as part of the denominator. Morbidity: Incidence and Prevalence RatesThe two principal types of morbidity rates, or rates of illness, in public health are incidence rates and prevalence rates. Incidence rates describe the occurrence of new cases of a disease (e.g., tuberculosis, influenza) or condition (e.g., teen pregnancy) in a community over a period of time relative to the size of the population at risk for that disease or condition during that same time period. The denominator consists of only those at risk for the disease or condition; therefore, known cases or those not susceptible (e.g., those immunized against a disease) are subtracted from the total population (Table 5-2). Incidence rate=Number of new cases or events occurring in the population in a specified periodPopulation at risk during same specified period×k  TABLE 5-2 Examples of Rate Calculations
The incidence rate may be the most sensitive indicator of the changing health of a community because it captures the fluctuations of disease in a population. Although incidence rates are valuable for monitoring trends in chronic disease, they are particularly useful for detecting short-term acute disease changes—such as those that occur with infectious hepatitis or measles—when the duration of the disease is typically short. If a population is exposed to an infectious disease at a given time and place, the nurse may calculate the attack rate, a specialized form of the incidence rate. Attack rates document the number of new cases of a disease in those exposed to the disease. A common example of the application of the attack rate is food poisoning; the denominator is the number of people exposed to a suspect food, and the numerator is the number of people who were exposed and became ill. The nurse can calculate and compare the attack rates of illness among those exposed to specific foods to identify the critical food sources or exposure variables. A prevalence rate is the number of all cases of a specific disease or condition (e.g., deafness) in a population at a given point in time relative to the population at the same point in time. Prevalence rate=Number of existing cases in population at a specified point in timePopulation at same specified point in time×k When prevalence rates describe the number of people with the disease at a specific point in time, they are sometimes called point prevalences. For this reason, cross-sectional studies frequently use them. Period prevalences represent the number of existing cases during a specified period or interval of time and include old cases and new cases that develop within the same period of time. The following factors influence prevalence rates: the number of people who experience a particular condition (i.e., incidence) and the duration of the condition. A nurse can derive the prevalence rate by multiplying incidence by duration (P = I × D). An increase in the incidence rate or the duration of a disease increases the prevalence rate of a disease. With the advent of life-prolonging therapies (e.g., insulin for treatment of type 1 diabetes mellitus), the prevalence of a disease may increase without a change in the incidence rate. Those who survive a chronic disease without cure remain in the “prevalence pot” (Figure 5-4). For conditions such as cataracts, surgical removal permits many people to recover and thereby move out of the prevalence pot. Although the incidence has not necessarily changed, the reduced duration of the disease lowers the prevalence rate of cataracts in the population. Morbidity rates are not available for many conditions because surveillance of many chronic diseases is not widely conducted. Furthermore, morbidity rates may be subject to underreporting when they are available. Routinely collected birth and death rates, or mortality rates, are more widely available. Table 5-2 provides examples of calculating selected rates. |
Risk for violent behavior can be identified using the epidemiological triangle 3 components
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