Epidemiologic Mind : Measurements of Morbidity and Mortality
Measurement of Health
Epidemiology is mainly a quantitative science. Measures of disease
frequency are the basic tools of the epidemiological approach. Health
status of a community is assessed by the collection, compilation, analysis
and interpretation of data on illness (morbidity),death (mortality), disability and utilization of health services.
The most basic measure of disease frequency is a simple count of affected
individuals. Such information is useful for public health planners and
administrators for proper allocation of health care resources in a
particular community. However, to investigate distributions and
determinants of disease, it is also necessary to know the size of the
source population from which affected individuals were counted. One of the
central concerns of epidemiology is to find and enumerate appropriate
denominators in order to describe and compare groups in a meaningful and
useful way. Such measures allow direct comparisons of disease frequencies
in two or more groups of individuals.
Ratios, proportions, and rates
Ratio
A ratio quantifies the magnitude of one occurrence or condition to another.
It expresses the relationship between two numbers in the form of x: y or x/y X k
Example:
-The ratio of males to females (M:F) in Ethiopia.
-The ratio of male malaria patients to female malaria patients
Proportion
A proportion quantifies occurrences in relation to the populations in which
these occurrences take place. It is a specific type of ratio in which the
numerator is included in the denominator and the result is expressed as a
percentage.
Example: The proportion of all births that was male
Rate
Rate is the most important epidemiological tool used for measuring diseases. Rate is a special form of proportion that includes time. It is the measure that most clearly expresses probability or risk of disease in a defined population over a specified period of time, hence, it is considered to be a basic measure of disease occurrence. Accurate count of all events of interest that occur in a defined population during a specified period is essential for the calculation of rate.
Example: The number of newly diagnosed pneumonia cases in 1999 per 1000 under five children.
Measurements of Morbidity
Morbidity rates are rates used to quantify the occurrence of disease. Measures of morbidity include incidence, period prevalence, and point prevalence rates.
Incidence Rate
The incidence of a disease is defined as the number of new cases of a disease that occur during a specified period of time in a population at risk for developing the disease.
Incidence rate = Number of new cases of a disease over a period of time X K
The critical element in the definition of incidence is new cases of disease. Because incidence is a measure of new events (i.e. transition from a non-diseased to a diseased state), incidence is a measure of risk. The appropriate denominator for incidence rate is population at risk but knowing the population at risk is difficult at this level. Hence, total population can be used as a denominator. Another important issue in incidence is the issue of time.
Read Also : Epidemiologic Mind: Measure Mortality
For incidence to be a measure of risk we must specify a period of time and we must know that all of the individuals in the group represented by the denominator have been followed up for that entire period. The choice of time period is arbitrary: We could calculate incidence rate in one week, one month, one year, 5 years, and so on. Incidence rates can be used to make statements about the risk of disease. If the incidence rate of a certain disease is high in one area, then the risk of acquiring that disease by other healthy individuals will be high.
Example. In Ginbot 1995 there were 50 new cases of relapsing fever in “Kebele X”. The average total population of “Kebele X” was 5000. Calculate the incidence rate of relapsing fever in “Kebele X” in Ginbot 1995.
That means out of every 1000 people living in “Kebele X”, 10 of them acquired relapsing fever in Ginbot 1995. Another commonly used measure of morbidity is attack rate. Attack rate is a type of incidence rate which is mainly used during epidemics.
On Tir 7, 1995, 100 people were invited by Ato Alemitegnaw for dinner. All of them ate the food that was served for dinner. The next day (Tir 8, 1995) 90 of the 100 people who ate that food developed diarrhea. Calculate the attack rate of diarrhea which occurred on Tir 8, 1995.
Attack rate = 90/100 X 100 = 90 cases of diarrhea per 100 people
That means out of 100 people who ate the food served by Ato Alemitegnaw, 90 of them developed diarrhea on Tir 8, 1995.
Uses incidence rate
Incidence rate is important as a fundamental tool for etiologic studies of diseases since it is a direct measure of risk. If the incidence rate is significantly higher in one area, then the cause of that disease can be systematically searched.
Prevalence rate
Prevalence rate measures the number of people in a population who have a disease at a given time. It includes both new and old cases. The major type of prevalence is point prevalence rate.
Point Prevalence rate : measures the proportion of a population with a certain condition at a given point in time. Point prevalence rate can be determined by conducting cross-sectional study.
Example: One health extension worker conducted a survey in one of the nearby elementary schools on Hidar 10, 1996 to know the prevalence of trachoma in that school. The total number of students in that school was 200. The health extension worker examined all the 200 students for trachoma. Hundred students were found to have trachoma.
Calculate the point prevalence rate of trachoma for that school.
That means 50 % of the students in that elementary school were affected by trachoma on Hidar 10, 1996.
Uses of prevalence rate
- Planning health facilities and human resource
-Monitoring chronic disease control programs like tuberculosis control program
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