Mortality is death. The frequency of death and the number of people who die is a measure of health of a community. Rates and ratios are frequently used for measuring mortality. Example are:
1. Crude death rate =– Number of deaths during the year/Mid – year population==1000
2. Specific death rate for disease
Number of deaths due to disease during a calendar year
mid – year population
3. Case fatality rate (ratio)
Total number of deaths due to a particular disease Total number of cases due to the same disease
It is defined as ‘any departure or deviation, from a state of physiological well – being’. Morbidity may be a sickness, illness or disability. Morbidity is also measured in terms of rates and ratios. The two important measurements of disease frequency (i.e the rate at which disease occurs) are incidence and prevalence.
Incidence rate is defined as ‘ the number of NEW cases of a specific disease occurring in a defined population during a specified period of time. It is given by the formula:
Number of NEW cases of specific diseases during a given time period
Population at risk
For example: The population of a particular year is 20,000. The number of new cases of a disease is 500. The incidence will be 1000 = 25 per 1000 per year.
Incidence rate refers 1. only the new cases. 2. during a particular period (usually one year) 3. a specified population (population at risk).
Uses of incidence rate: 1. to control the disease 2. for research in etiology, pathogenesis and distribution of diseases.
The term disease prevalence refers to all current cases (old and new) in a given population at a particular point of time or over a period of time. The term incidence refers only to new cases, but prevalence refers to both new and old cases. Prevalence is classified into two types:
1. Point prevalence: It refers to the number of all current cases (old and new) at a particular point of time (e.g. particular day or particular week.)
2. Period prevalence: It refers to the number of all current cases (old and new) during a particular period of time (e.g. period of one year)
Relationship between prevalence and incidence
Prevalence depends on 2 factors; the incidence and duration of illness. This relationship is expressed as:
Prevalence = incidence duration
P = I D
Incidence (new cases) = 20 cases per year per 1000 population
Duration of the disease = 5 years
Prevalence = 20 5 = 100 per 1000 population
Incidence and prevalence can be exemplified with a tub provided with an inlet tap and outlet. The amount of water pouring through the inlet tap is the incidence (new cases). Prevalence is the amount of water in the tub at any point of time or period of time, (old and new cases). The outlet is the cases died due to the disease or recovered from the disease.