MEASUREMENT OF DISEASE OCCURRENCE IN A POPULATION

Statistically, the measures and/or determination of disease occurrence in a given population are usually expressed practically as rates or fractions which are stated as per the number of people in the population (for example 10, 100, and 10,000). The fraction is generally expressed as a numerator (which is the number of persons with the disease) and a denominator (which is the number of persons at risk).

These measures are important epidemiological tools which an epidemiologist uses to unravel the immediate and remote causes of a particular disease in a population. In epidemiological survey, ratios and rates are used to characterize a particular population by race, sex, age, occupation and their different level of exposures to an infection in the community.

A variety of ways are used by researchers to summarize data emanating from an epidemiological survey into a practical and applicable format used for implementation. Some of these means by which the frequency of a given disease can be evaluated include: incidence rate, prevalence rate, morbidity rate and mortality rate. These factors (incidence, prevalence, mortality, morbidity) are used for measuring disease occurrences and their subsequent distribution within a community.

It is noteworthy that deductions from statistical analysis in any epidemiological study are part and parcel of the study of epidemiology in view of the fact that the data emanating from epidemiological surveys are usually subject to bias and fluctuations. In addition, the determination of and conclusions that are generated from the incidence rate, morbidity rate, prevalence and mortality rates are utilized by public health professionals and policy makers in the planning, budgeting and adequate delivery of medical, sanitation and treatment services in a particular population so as to contain the spread of a disease and its pathogen in a community.  

Incidence rate (IR): Incidence rate is the number of new cases of a disease amongst individuals in a community at a particular time period. IR measures the frequency of addition of new cases of a disease to a population. It is generally used to look at the rate at which new cases of a particular disease occur in a population; and it can be used to study the aetiology of a disease. Incidence rates are the most common way of measuring and comparing the occurrence of a given disease in a community.

The incidence rate is given as:

Number of new cases of disease in a given time period/Total number in population at risk at that time period

Prevalence rate (PR): Prevalence rate is a measure of the new and old cases of a disease reported in a community at a given time period. It is the total number of persons infected by a disease in a population at a given time irrespective of when the disease occurred. Prevalence rate takes into consideration not just new cases of a disease (as is the case in IR) but also the existing disease cases; it is dependent on the duration of the disease and the IR of the disease cases. The PR is descriptive, and it is usually used to describe the degree of disease in a given community. PR is given as:

Number of cases of disease (old & new) in a given time period/Total number in population in that time period

Morbidity rate: Morbidity rate is the number of cases of a particular disease in relation to the entire population under study. It is used to describe the public health statusof a community since most diseases have a comparatively low death rate. Morbidity rate is an incidence rate that reveals the frequency of new cases of a disease in a particular population and over a specified time period.

It is given as follows:

Number of new cases of a disease during a specific period/Number of persons in the population

Other morbidity frequency measures apart from incidence rate include: attack rate and prevalence rate.

Mortality rate: Mortality rate is a measure of the frequency of occurrence of death (resulting from a given disease) in a particular population over a specific period of time. Mortality rate is the association of the number of persons that died from a given disease in a population to the total number of cases of the disease.

The mortality rate is given as:

Number of deaths due to a given disease/Size of the total population with the same disease

Other mortality frequency measures include: neonatal mortality rate, maternal mortality rate, infant mortality rate and crude death rate.

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