Epidemiology is the science of study of distribution on the various determinants which play a significant role in the health related issues in a specified population and the implications of the study results in order to control the health problems. Descriptive epidemiology is intended to classify the details of a disease based on person, place and timing. The methods of causal reasoning, which is used for testing the hypothesis is carried out mainly with the inputs derived out of the descriptive epidemiology. The descriptive epidemiology intend to describe a disease as such.

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map of epidemiology of measles US

Image: “Epidemiology of measles: admission rates per 1,000 per annum in white enlisted men in the U.S. from April 1917 to December.” by Otis Historical Archives of “National Museum of Health & Medicine” (OTIS Archive – http://www.flickr.com/photos/medicalmuseum/5616561595/in/set-72157614294614418/. License: CC BY 2.0

Classification of Epidemiology

Epidemiology is classified into three types, namely descriptive epidemiology, analytical epidemiology and experimental epidemiology.

With regards to the type of studies, descriptive epidemiology basically concentrates on the correlation studies at the population level and the case series, case reports and cross-sectional studies at the individual level. Analytical studies in turn is divided into the observational studies and the experimental studies, which in turn are divided into further types.

Correlation and case report

In the correlation study the general characters of the population are measured. This is essential for the purpose of getting ideas for conducting the further studies. The case report is where we report either the individual case or that of the group of cases with that of the similar feature (either as a diagnosis or some other factor).

Statistical association studies could not be done using the case report. The cross section study is one where we just measure the disease along with that of the exposure simultaneously in the population. Though the prevalence of the disease can be found, it would be difficult to find the incidence.

Differences between the descriptive and analytical epidemiology

Descriptive epidemiology is mainly applied when the knowledge about a particular disease is little known and it is intended only to illustrate a potential association. In addition, it mainly relies on the data which were previously present.

Analytical epidemiology, on the other hand, is applied when the insight about a particular disease is already available pre-hand. The key is in evaluating the causality associations for the disease.

Advantage of descriptive epidemiology

The cost incurred in conducting descriptive epidemiologic studies is relatively very less and inexpensive when compared to analytical studies. The process entirely is less time-consuming.

After conducting these studies, the general factors about the disease such as the highest and lowest rate of the occurrence of the disease within the population, the temporal patterns of a particular disease are unearthed to a greater extent. The descriptive epidemiology data will be assisting the government in a very big scale in their decision of resource allocation for tackling the health problem on the priority basis.

Overall detail

The factor “person” consists of describing a person in terms of race, age and sex. The “place” consists of describing a disease in terms of geographical location. The difference in the prevalence of the disease could be due to the same factor in the two places. The time consists of distribution of the disease based on time span in which it occurs to a large extent.

The time factor represents secular (there is a long term trend the disease might occur), periodic, seasonal and epidemic. The cyclic is the alteration which keeps occuring in itself like that of a cycle. The variation which occurs by means of the seasonal factor such as the time and year constitutes this.

The disease is classified as epidemic if in a particular set location, the occurrence of the incidence of the disease goes above that of the expected. The other terminologies based on this include that of the endemic (the occurrence is similar to that of what would be expected from the habitual occurrence), epidemic (the clear excess in the occurrence of a particular disease) and pandemic (the diseases affecting the worldwide population).

The diagnosis of cholera as the pathological agent by John Snow signifies the importance of the place in the descriptive epidemiology.

Incidence and prevalence

In a population at risk for a particular disease, the proportion of all the new cases constitutes the incidence. In a population for a particular disease, the proportion of all present cases constitutes the prevalence. The mortality rate on the other hand is a marker of the proportion of the population which, taken over a particular lineated time period, has died.

In a defined population under study, the frequency at which a particular event occurs constitutes the rate. Whereas, when we divide a particular value with another value and obtain a third value, this constitutes the ratio and in many of the cases constitutes an entirely different value. This will give the perspective of both the variables under the study and would help to interfere the changes at a much interpretable.


In simple English terms, this represents the occurrence of a new event. It almost represents the same in the statistics. The time period is kept fixed and within that span, the number of the new instances of the particular disease under the study constitutes the incidence of the disease. It is form of the rate.

Incidence density and cumulative incidence

Sometimes the framework within which we study a disease might keep on vary. This brought about the concept of the person time. This is basically like a multiplication factor which represents the rate of an occurrence of a new case in a particular disease (which is nothing but the incidence) over the varied time length. This is given with the name of the incidence density.

Incidence density = Number of new cases / (Population at risk * Duration of risk)

If for example you follow 10 people and among them, 5 people get diagnosed for malaria. Then first you add up all the varying time points and then you divide the 5 by total time period for all the patients (this is represented by the patient years).

When we have to have a specific time period rather than varying time period, the concept of cumulative incidence occurs. In this we see the rate of all the cases which occurred new over the time period of interest.

Cumulative incidence rate = Number of new cases / Population at risk at the beginning of the study

In Egypt in a particular place, there are totally 100,000 people followed up for a period of 5 years and at the end around 1,000 people developed the disease. The incidence in this example is equivalent to 0.002.


In a particular time point, the number of people who have a particular disease in the population under consideration constitutes the prevalence. Rather than the incidence, one of the important differences is that it constitutes the ratio and takes into account both the new cases and also the already present cases for obtaining the final value.

Case fatality ratio and proportional mortality ratio

It should be recognised that not all diseases are fatal. When we get affected by a particular disease, the probability of dying from this disease constitutes the case fatality ratio.

Secondly, though a particular disease may be fatal, the overall impact of the death when compared to that of all the fatality in the population might be low. The proportion of the death from a particular disease when compared to that of the whole population constitutes the proportional mortality ratio.

Prevalence based on the view of the time

The query about the disease might naturally come as the query about the occurrence of the disease at a particular time point or over a period. These constitute the point and the period prevalence.

When the ratio of the diseased people over that of the population is taken over a particular specific time point, it represents the point prevalence whereas the proportion of the diseased people over that of the population over the period of time is the period prevalence.

Mortality Rate Concepts

Crude death rate

As the name suggests, we just make a crude estimate of the death rate in the population. In a single year, the total death in number which occur for every 1,000 people constitutes the crude death rate.

Perinatal mortality rate

The word “peri” means in and around. The natal is the event of child birth. The perinatal mortality represents the mortality of neonates (WHO definition: within 7 days after birth) along with that of the fetal death (greater than 28 week of the intrauterine life, other definitions have been used) for every 1,000 births.

Maternal mortality rate

For every 100,000 live births which occur, the number of the maternal deaths constitutes the maternal mortality rate and it represents the gross health care system of the society. Many countries take this as the main decisive factor in improving health care.

It is represented as a lakh birth, as childbirth is a natural process and the mother is generally looked up for the survival during the event. The untimely death of the mother is rare. So a considerable child birth event is taken for obtaining a comparable value.

Infant mortality rate

For every 1,000 live births, the number of death of the children less than one year of age constitutes the infant mortality rate. This is also one of the sensitive indicators in the health care setup of a particular country. Every country aspires to keep the infant mortality as well as the maternal mortality rate to that of the bare minimum.

Standardised mortality ratio

For every population there is a set mortality based on age, gender and similar other factors when the mortality is compared as the proportion of the number of the deaths between a studied population with that of the standard population. This constitutes the standardised mortality ratio.

Case fatality ratio and proportion mortality ratio

This can be well understood by a simple example. When you get a common cold, is there a need to panic? Though there might be some trouble, it is unlikely in all the circumstances that a common cold might lead to the death of the patient.

But at the same time if a person is diagnosed with fulminant hepatic failure, it is absolutely essential to get admitted into a hospital and get treated. This is important as the fatality is more in this disease. This is compared in the case fatality ratio. It represents the probability of dying with that of a particular disease after having contracted it.

Case fatality rate = Number of deaths in the disease under consideration / Total number of people who got the disease

The second concept can be understood by the fact that though some diseases are very dangerous and may cause mortality, the total composition that the disease is less compared to that of the other disease. On the other hand, though, the case that a disease might be less fatal is more common in the occurrence. This is the proportional mortality ratio and it represents that out of all the death, what number of the person died of the disease.

Proportional mortality rate = Number of deaths in the disease under consideration / total number of deaths ∈ population

Population Pyramid

In a population, the distribution of the ages might be haphazard or it may follow the normal pattern. The difference also lies in the sex. The population pyramid gives an opportunity to view both the age as well as the gender differences in that of the population in a single figure. This helps in appreciating the differences to a better extent.

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