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An epidemiological study usually involves two important steps that must be put into consideration prior to the investigation of disease/infection outbreak. The first step in any epidemiological study is to define the hypothesis to be tested. The hypothesis must take into consideration the description of the exposure and possible outcome of the disease. The second step is to determine the type of epidemiological study design to adopt, and which will be most appropriate to test the already defined hypothesis. In the course of anticipating any epidemiological survey, the type of epidemiological study design adopted by an epidemiologist (or public health workers, researchers or those involved in disease surveillance) is a fundamental decision that must be made and chosen carefully. This is because there are varieties of available study designs for epidemiological investigations, and each of this study designs differ in their merits and demerits. Epidemiological investigations can be directed towards revealing the determinants of a disease or describing the way and manner in which the disease/infection is being distributed within a defined population/community.

The different types of available and applied epidemiological studies reflect the diver’s ways or means of gathering information or data in the scenario of a disease/infection outbreak. The preference of one type of study design over another is usually centered on the ethical issues surrounding the anticipated study, the validity of the study, its efficiency and research questions to be addressed. It should be noted that disease/infection does not just happen in a space or vacuum but rather a host (i.e. an individual) and other environmental factors are paramount to the establishment of a disease state in a given population/community. Because of this, epidemiologists therefore rely on systematic or logical approach in the investigation of disease distribution within a community. Thus, epidemiological study designs allows for the determination of disease agents in a classified manner while putting in place appropriate preventative measures to forestall disease emergence and spread to unexposed members of the population/community. As an epidemiologist or researcher anticipating an epidemiological study, it is very critical to take into consideration the most appropriate epidemiological study design to be adopted, and this must encompass factors such as the cost of the study, its practical feasibility, duration of the study and the expected data or result. Based on these facts, epidemiological studies can be broadly divided into two categories which are Observational Epidemiological Study and Experimental Epidemiological Study.


Observational epidemiological studies are non-experimental epidemiological investigations that involves no intervention by the researcher (in this case the epidemiologist) other than carrying out medical and laboratory examinations and probably asking questions about the issue at hand. In this type of epidemiological study, nature is allowed to take its full course while the researcher only observes passively and measures some disease parameters (factors) but does not interfere. This type of study design allows one or more groups of subjects and their disease characteristics to be observed and described/analyzed. Observational epidemiological study investigates the cause, prevention and possibly the treatment of a disease outbreak. In observational study, the investigator is merely an observer who only records potential factors of the disease/infection and the outcome of the poor health of the community. He/she notes the number of exposed and unexposed individuals in the population and, also the proportion of persons who has developed or has not at all developed the expected disease outcome.

Observational studies provide data that explains the occurrence of disease and determinants of disease evolution in order to control the disease and plan the prospects of the population’s health care needs. Though observational epidemiological studies can be used to study the effects of a broader range of exposures to a disease/infection, it is still not without some limitations. Firstly, investigators who opt to use observational study designs have limited or no complete control over some alarming factors bothering on their study – as they only observe passively and allow nature to take its course. Secondly, the observed group in the investigation may differ in many other features other than that being studied. Observational study does not require any ethical consideration before it can be undertaken, and it takes advantage from the fact that individuals of a population/community are exposed to certain disease/infection following their personal habits (e.g. smoking, drinking), occupation, time and place of work amongst other factors. The relationship between a given exposure or risk factor for the acquisition of a disease/infection in a community/population, and its outcome can be studied in several ways. Observational epidemiological studies therefore can be either descriptive or analytical study design.


Descriptive epidemiological studies look at the frequency and distribution of a disease/infection within a population. It is usually the first step in any epidemiological survey and, it only describes the occurrence or outcome of a disease in a community/population. In a descriptive study, the general features of an infection in a population are typically described in relation to the time, place and person getting the disease and, in what rate or frequency the disease is occurring in the community under investigation. Place, time, and person are very important epidemiological variables that are looked out for in any descriptive epidemiological study and, which allows a public health worker or an epidemiologist to decipher by descriptive approach the extent of a disease being investigated in a population, know the number of individuals that are easily contaminated, and most of all the proportion of persons at risk of acquiring the disease/infection. Such knowledge help health policy makers to effectively plan interventional measures and other preventive, treatment and educational programs that will help to reduce the scourge of the prevailing disease within the community.

Descriptive epidemiological studies are deficient and limited in their usage in that they only look at the features of some group of people with a given disease/infection without actually comparing them with some other reference or non-affected individuals of the population. Epidemiological studies that are descriptive in design are usually used for the formulation and development of hypothesis that are normally tested with a more detailed study (e.g. analytical epidemiological study).

Descriptive epidemiological study may include:

  1. Case reports
  2. Cross-sectional (Prevalence) studies
  3. Case series
  4. Ecological (Correlational) studies


Analytical epidemiological study is a more detailed and all encompassing epidemiological study than the descriptive epidemiological study. An analytical study describes the association between exposure and outcome (disease). It is meant to test the hypothesis of a descriptive epidemiology. Analytical study investigates the cause of a disease by studying how exposure of individuals relate to the disease (i.e. it studies the relationship between a disease and its determinants). Virtually all epidemiological studies (except observational studies) are analytical in nature. In analytical epidemiological study, the epidemiologists or researcher systematically investigates whether or not the risk of a disease is different for exposed and unexposed persons in relation to a particular variable. Analytical epidemiological studies deal more with the “how” of a disease/infection in a population at a given time. It looks at the cause of a disease, the process of transmission (i.e. the how), and the relationship of the different factors or variables of the disease (i.e. the why) in a population/community. This study usually involves some series of laboratory procedures in order to supplement epidemiological studies. 

Analytical epidemiological study may include:

  1. Case-control (Retrospective) studies
  2. Cohort (Prospective) studies


Case reports are epidemiological studies that consist of the careful and thorough or comprehensive description of a patient’s health or disease summary which is usually given by the physician in charge of that patient. It is the most fundamental type of descriptive epidemiological study obtained from an individual in the investigation of a disease outbreak. A case report involves a single case (i.e. one subject) with respect to a patient and, it describes unusual characteristics of the case or disease at the moment. It does not measure disease frequency and risk, and this type of descriptive epidemiological study is usually inexpensive to perform, speedy to carry out, and have no follow-up period. Case reports are entirely descriptive and as such it cannot be effectively used to make conclusions about the general population of individuals (in this case patient’s) that are harbouring the disease being investigated. The sole purpose of a case report is to generate hypothesis that will further be used to address a disease outbreak properly.   


Case series is an expanded case report epidemiological study. It is the qualitative study of the experience (exposure) of a small group of patient’s or individual patient to a disease and, that has a related medical diagnosis. Data from a case series epidemiological study are used to analytically investigate the underlying characteristics of a disease in any population/community of people. Case series epidemiological study describes the characteristics of a number of patient’s (more than a single case) with a given disease in a population. Such studies are quick, inexpensive, and the cause-effect relationship of a case series study (just as in case report study) cannot be interpreted. A group of patient is examined in line with factors of a given disease, and case series studies also help to generate hypothesis. Case series studies can either be case-control studies or cohort studies.


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Nelson K.E and Williams C (2013). Infectious Disease Epidemiology: Theory and Practice. Third edition. Jones and Bartleh Learning. 

Riedel S (2004). Biological warfare and bioterrorism: a historical overview. BUMC Proceedings, 17:400-406. 

Rothman K.J, Greenland S and Lash T.L (2011). Modern Epidemiology. Third edition. Lippincott Williams and Wilkins, Philadelphia, PA, USA.

Schneider M.J (2011). Introduction to Public Health. Third edition. Jones and Bartlett Publishers, Sudbury, Massachusetts, USA.

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