Introduction 

 

Measurement generates numerical data or categorical data that may be structured to reveal patterns from which logical inferences can be made. Epidemiology focuses, among other things, on the measurement of mortality and morbidity in human populations. The first requirement is, therefore, the definition of what is to be measured and the establishment of criteria or standards by which it can be measured. This is not only a prerequisite of epidemiological studies but also one of its goals. Here we have given basic measurements that are used in epidemiology. 

The scope of measurements

The scope of measurement in epidemiology is very broad and unlimited and includes the following:

• Measurement of mortality
• Measurement of morbidity
• Measurement of disability
• Measurement of natality
• Measurement of the presence, absence or distribution of the characteristics or attributes of the disease
• Measurement of medical needs, health care facilities, utilization of health services, and other health-related events
• Measurement of the presence, absence or distribution of the environmental and other factors suspected of causing the disease
• Measurement of demographic variables

1. Frequently used measures of Mortality (CDC)

S.N	Measure	Numerator	Denominator	Calculated in
1.	Crude death rate	Total number of deaths during a given time interval	Mid-interval population	1,000 or 100,000
2.	Cause-specific death rate	Number of deaths assigned to a specific cause during a given time interval	Mid-interval population	100,000
3.	Proportionate mortality	Number of deaths assigned to a specific cause during a given time interval	Total number of deaths from all causes during same time interval	100 or 1000
4.	Death-to case ratio	Number of deaths assigned to a specific cause during a given time interval	Number of new cases of same disease reported during the same time interval	100
5.	Neonatal mortality	Number of deaths among children <28 days of age during a given time interval	Number of live births during the same time interval	1,000
6.	Infant mortality rate	Number of deaths among children <1 year of age during a given time interval	Number of live birth during the same time interval	1,000
7.	Maternal mortality rate	Number of deaths assigned to pregnancy-related causes during a given time interval	Number of live births during the same time interval	100,000

      2. Some frequently used measures of Morbidity

S.N	Measure	Numerator	Denominator
1.	Incidence proportion	Number of new cases of disease during specified time interval	Population at start of interval
2.	Incidence rate	Number of new cases of disease during specified time interval	Summed person-years of observation or average population during the time interval
3.	Secondary attack rate	Number of new cases among contacts	Total number of contacts
4.	Point prevalence	Number of current cases (new and preexisting) at a specified point  in time	The population at the same specifies the point in time
5.	Period prevalence	Number of current cases (new and preexisting) over a specified period of time	Average or mid-interval population

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Despite all these measurements, there are still potholes to be covered. For example, measurement of the psycho-social aspects of health and disease are not yet covered. The basic requirements of measurements are validity, readability, accuracy, sensitivity, and specificity. Finally, measurement errors are unavoidable, no matter where and by whom measurements are taken.

The basic tools of measurement in epidemiology are:

1. Rates:  A rate in epidemiology is a measurement of how frequently an event happens in a specific population over a certain time frame. Rates are particularly helpful for comparing illness frequency in different places, at different periods, or among distinct groups of people with possibly different sized populations because they put disease frequency in the context of population size; in other words, a rate is a measure of risk.
2. Ratios: A ratio is a comparison of any two values or the relative magnitude of two quantities. One interval- or ratio-scale variable is divided by the other to compute it. No relation must exist between the numerator and denominator. So, one may contrast apples with oranges or apples with the quantity of doctor appointments.
3. Proportions: A proportion is the comparison of a part to the whole. It is a particular kind of ratio where the denominator also contains the numerator. The percentage of clinic patients who tested positive for HIV or the proportion of people under 25 years old in the population may both be expressed using proportions. A percentage, a fraction, or a decimal can all be used to represent a proportion.

Some more references: SAGE Publication