Basic Measurements In Epidemiology

By Salwa M Osman

The first requirement of measurement is definition of what is to be measured and establishment of criteria or standard by which it can be measured .

The definition Should be: Acceptable.

Applicable to its use in large population.

Valid

Clear definitions help to minimize errors in classification of data.

The scope of measurements in epidemiology is very broad and unlimited and include the following:

Measurements of mortality. Measurements of morbidity. Measurements of disability.

Measurements of the presence ,absence or distribution of the characteristics or attributes of the disease.

Measurements of natality. Measurements of medical needs ,health care

facilities , utilization of health services and other health related _events

.Measurements of the presence ,absence or distribution of the environment and other factors suspecting of causing the diseases.

Measurements demographic variables.

The basic requirements of measurements are: Validity. Reliability. Accuracy. Sensitivity. Specificity.

Tools Of Measurements:

RATE.

RATIO.

PROPORTION.

RATE:

The rate measures the occurrence of some particular events in a population during a given time period .

The rate is a statement of the risk of developing a condition .

The rate indicates the change in some events that takes place in a population over a period of time .

Death rate:

Number of deaths in one year x 1000Mid-year population

It comprises these elements : Numerator Denominator(the numerator is a component of the

denominator) Time specification (usually a calendar year.) Multiplier

The rate is expressed per 1000 or some other round figure (10.000,100.000) selected according to convention to avoid fractions.

Rate categories:

Crude rates.

Specific rates.

Standardized rates.

RATIO:

Ratio is expresses a relation in size between two random quantities .

The numerator is not component of the denominator

The ratio is a result of dividing one quantity by another.

It expressed in form of :

X:Y OR X/Y

For example: The ratio of WBCs relative to the RBCs is 1:600 or 1/600

The proportion is a ratio which indicate the relation in magnitude of a part of the whole

The numerator is always included in the denominator

The proportion is usually expressed as percentage.

PROPORTION:

For example: the number f children with scabies at a certain time x100

the total number of children in the village at the same time

Concept of numerator:

It refers to the number of times an event has occurred in population ,(e.g. sickness ,death ,episodes of sickness ) during a specific time- period .

Concept of denominator :

The numerator has little meaning unless it is related to the denominator .

The denominator may be: Related to the population. Related to the event.

Related to the population: Mid –year population.Because the population size changes daily due

to birth , death and migration.It refers to the population estimated as on first

of July an year .

Population at risk:The term is applied to those to whom an event

could have happened whether it did or not .Sometimes it may be necessary to exclude

people because they are not at risk.

PRSON-TIME:As in cohort study persons may enter the study

at different times in such cases denominator is a combination of persons and time .

The most frequently used is person-years.For example if 10 persons remain in the study

for 10 years it will be 100persons year.

PERSON-DISTANCES: Passenger-mile SUBGROUB OF THE POPULATION: e.g. age ,sex ,occupation , social class ,etc…

Related to total event: In some instance the denominator may be

related to the total event as in accident the number of accident per 1000 vehicle will more useful than total population many of them may be not using vehicle.

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