Epidemiology is the science of studying distribution with regards to the various determinants that play a significant role in health related issues within a specified population as well as the implications of the study results in order to control health problems. Descriptive epidemiology is intended to classify the details of a disease based on person, place and timing factors. The methods of causal reasoning, which are used to test the hypothesis are carried out mainly using inputs derived out of descriptive epidemiology. The various aspects of descriptive epidemiology are covered in the following article.

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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 can be classified into three classes, 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 are divided into observational studies and experimental studies, which in turn are divided further into sub-types.

Correlation and case report

In a correlation study the general characteristics of the population are measured. This is essential so as to get ideas to conduct further studies. The case report is where we report the findings of either an individual case or those of a group of cases with similar features (either as a diagnosis or some other factor).

Statistical association studies can not be done using the records of a case report. A cross section study is one where we only measure the disease prevalence along with its level of exposure simultaneously within a population. Though the prevalence of the disease can be concluded, it can be difficult to find the incidence.

Differences between the descriptive and analytical epidemiology

Descriptive epidemiology is mainly applied when knowledge about a particular disease is not well known and it is intended only to illustrate a potential association. In addition, it mainly relies on data which was previously present.

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

Advantage of descriptive epidemiology

The cost incurred in conducting descriptive epidemiological studies is relatively very low and inexpensive compared to that of analytical studies. The whole process is less time-consuming.

After conducting these studies, the general factors about a disease such as the highest and lowest rate of occurrence within the population and the temporal patterns of a particular disease are uncovered to a greater extent. The descriptive epidemiology data assists the government by a huge amount in their decisions of resource allocation in order to tackle the health problem on a priority basis.

Overall detail

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

The time factor represents secular (the disease might occur over a long period of time), periodic, seasonal and epidemic. The cyclic is the alteration which keeps occurring by itself in a cycle. The variation which occurs due to seasonal factors such as time and year constitute this.

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

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

Incidence and prevalence

In a population at risk for a particular disease, the proportion of all new cases constitutes the incidence. In a population affected by 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,taken over a particular lineated time period, which 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, which constitutes the ratio and in many cases constitutes an entirely different value. This gives the perspective of both variables under study and it helps to interfere with the changes so that they become more interpretable.

Incidence

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

Incidence density and cumulative incidence

Sometimes the framework within which we study a disease might keep on varying. 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 a varied time length. This is referred to as the incidence density.

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

If for example you monitor 10 people and, among them, 5 get diagnosed with malaria. To get the incidence density you first add up all the varying time points and then divide 5 by the total time periods of all patients (this is represented by a patient’s years).

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

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

In a particular location of Egypt, there are a total of 100,000 people monitored over a period of 5 years and at the end around 1,000 people developed the disease under study. The cumulative incidence rate in this example is equivalent to 0.002.

Prevalence

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 those already present in order to obtain the final value.

Case fatality ratio and proportional mortality ratio

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

Secondly, though a particular disease may be fatal, the overall impact on death by the disease compared to that of other diseases within a population might be low. The proportion of death from a particular disease when compared to that of the whole population constitutes the proportional mortality ratio.

Prevalence based on the view of 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 given period. These constitute the point and period prevalence.

When the ratio of the diseased people over that of the population is taken over a particular time period, 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 within a given population. In a single year, the total number of deaths that occur for every 1,000 people constitute the crude death rate.

Perinatal mortality rate

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

Maternal mortality rate

For every 100,000 live births that occur, the number of maternal deaths constitutes the maternal mortality rate and it represents the gross health care system of the society. Many countries use this as a decisive factor when improving health care services.

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

Infant mortality rate

The number of children deaths, for every 1,000 live births, less than a year old constitute 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 rate as well as the maternal mortality rate as low as possible.

Standardised mortality ratio

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

Case fatality ratio and proportion mortality ratio

This can be better understood through a simple example. When you get a common cold, is there need to panic? Though there might be a problem, it is unlikely in most circumstances that a common cold by itself will cause death to a patient.

But at the same time, if a person is diagnosed with fulminant hepatic failure, it is absolutely essential to get admitted to a hospital and get treated immediately. This is important as the fatality rate is higher in such a disease. This is comparable to the case fatality ratio. It represents the probability of dying from a particular disease after having contracted it.

Case fatality rate = Number of deaths due to the disease under consideration / Total number of people who contracted the disease

The second concept can be explained by the fact that though some diseases are very dangerous and may cause death, the total occurrences of the diseases might be lower compared to that of other diseases. On the other hand, though a disease may be less fatal compared to other diseases, it may be more common in terms of its occurrence. This is the proportional mortality ratio and it represents the number of people who die from a particular disease out of the total number of deaths.

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

Population Pyramid

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

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