Epi – on/upon ; demno – people ; logy – study . Epidemiological research is the systematic investigation of how, when, and where diseases occur in populations.


Epidemiological research methods are the methods that are used to identify and classify the origins and spread of epidemic diseases. It involves determining the source of the disease as well as the vector (population) by which it is spread, identifying what causes the disease (virus, bacteria, fungus, etc. ), and determining how to control the given disease.

An epidemiologist investigates the factors that contribute to disease (risk factors) and the factors that safeguard against disease (protective factors). To identify and learn about the causes of contagious diseases such as smallpox, typhoid, and polio, epidemiology was initially developed. As a branch of the field, it has expanded to include the study of factors related to non-transmissible diseases like cancer, and of environmental poisons like lead.  

Epidemiological studies can never demonstrate causation; that is, they cannot prove that a particular risk factor is responsible for the disease being studied. In epidemiological evidence, we can only demonstrate that this risk factor correlates (correlates) with a greater incidence of disease in the population exposed to that risk factor. An increase in correlation will increase the likelihood of an association, although it cannot prove causation.  

For example, the link between cigarette smoking and lung cancer was discovered by studying lung cancer rates among smokers and non-smokers. Compared to non-smokers, smokers have higher lung cancer rates. Does this prove that cigarettes cause lung cancer? No. It was necessary to expose animals to tobacco smoke and tobacco smoke extracts in order to prove that cigarette smoking causes lung cancer. The experiment was conducted under highly controlled conditions, with the only difference between the controls (animals not exposed to smoke) and the treated animals being smoke exposure. These studies supported the causal association between smoking and increased cancer risk.  


  1. whether the events have already happened (retrospective) or
  2. whether the events may happen in the future (prospective).

The most common studies are retrospective studies which are also called case-control studies. If an outbreak of disease is noted and the cause is unknown, or the disease is unusual within the population studied, a case-control study may be initiated.  


Epidemiology study designs serve to ease the researcher’s process of addressing a research question logically with the least amount of ambiguity possible.  


  1. Study of disease in its descriptive form.
  2. Case control using the diagnosed cases and confirmed control.
  3. Cohort study using exposed and unexposed cohort.
  4. Experimental study : removing he suspected cause in group 1 and not in others, then observing the cure for each group.


  • A historical study of the effects of population health and the emergence and fall of disease in the period may also yield useful projections into the future.
  • For community diagnosis, which considers the dimensions of health and disease among the population as well as their incidence, prevalence, and mortality, keeping in mind the fact that societies are changing and health issues are changing.
  • Studying the health services: how they work. The process begins with the determination of needs and resources, then moves on to identifying services in action and, finally, attempting an assessment. Various populations can be compared in such studies.
  • In order to estimate the likelihood and risk of disease in the individual based on the collective experience.
  • To complete the clinical picture by including all types of cases; relating clinical diagnosis to subclinical symptoms; and observing the secular change in disease’s nature and its manifestation in other countries.
  • Identification of syndromes from the distribution of clinical phenomena within sections of a population.
  • In the quest for the causes of health and disease, begin with identifying groups with high and low incidence rates, examine how these differences correlate with differences in lifestyles and put those notions to the test among group members.


  • Descriptive: Descriptive research primarily deals with describing the features of a specific demographic segment, rather than asking “why” a phenomenon occurs. Therefore, it only “describes” the research subject and does not address “why” it happens. So descriptive research is the process of answering research questions, designing the research, and analyzing the data on a topic. A research study is called observational because no variables involved in it are influenced in any manner. When the objective of the study is to identify characteristics, frequencies, trends, correlations, and categories, descriptive research is an appropriate method. It can be useful when the topic or issue isn’t well understood. Understanding how, when, and where something happens is vital to understanding why it happens.
  • Analytical: The purpose of this comparative study was to determine whether risk factors and outcome have a causal relationship. In analytical studies, associations are identified and quantified, hypotheses are tested, causes are identified, and the issue of whether an association exists is resolved. The probability of a relationship arising simply by chance is determined using statistical procedures. In an analytical study, two or more groups or sets of data are compared. There are two types: 1. Case-control studies 2. Cohort studies
    1. Case-Control Studies: Studying case-control instances is used to determine the extent to which different risk factors correlate with outcomes. Exposures are factors that affect the risk of disease. Studies in case-control settings are useful for identifying beneficial and harmful exposures. As the name implies, there are two groups of patients in a case-control study: cases and controls. Patients with a specific disease, condition, or disability are called cases. Patients without the disease are considered controls. Researchers typically select representative controls from the general population for the cases they are studying. Retrospectively, they check to see if any risk factors might have been presented to these patients in the past. Control group selection is one of the most critical elements of case-control study research. Case-control studies tend to exhibit recall bias since the study design is retrospective. Studying cases and controls is efficient, cost-effective, and often easier than reporting results. The design of this study is especially suitable for rare diseases with longer latency periods.
    2. Cohort studies: Initially, cohort studies classify patients into two groups based on their exposure status. The exposed and non-exposed groups are followed over time to see who develops the disease. The cohort study can be a retrospective or a prospective study. In a cohort study, you can directly estimate incidence since you start with exposed and unexposed patients, unlike a case-control study where you begin with diseased and non-diseased patients. A cohort study’s relative risk is its measure of effect. Cohort studies have very low recall bias, and multiple outcomes can be studied at the same time. In cohort studies, selection bias is one of the disadvantages. Cohort studies can be very time-consuming and expensive when studying rare diseases and outcomes that have long follow-up periods.
  • Experimental : It is any research that is conducted with a scientific approach, with one variable kept constant while the other is measured as the subject of the experiment. Sometimes the choice of a course of action is not supported by adequate data. The best way to learn the truth in such circumstances is to conduct experiments. The results of experimental research can help you make better decisions. 1. Randomized control trial 2. Community trial 3. Field trial
    1. Randomised control trial: Randomized clinical trials or randomized control trials (RCT) are considered the gold standard when it comes to designing studies. In an RCT, the subject is assigned randomly to either a control group or an experimental group. RCTs avoid confounding and minimize selection bias through randomization. Thus, it becomes possible for the researcher to isolate the effect of an intervention through the use of similar experimental and control groups. During the experiment, subjects are exposed to an agent which can either cause, prevent or treat a disease. In a control group, no treatment is given, a placebo is administered, or another standard of care is administered. Next, we follow up on the groups prospectively to see who has developed the desired outcome. In RCTs, randomized control trials can be expensive, and researchers face issues with the integrity of randomization caused by refusals, dropouts, and crossovers.
    2. Community trial: Trials of communities (also called community interventions) are usually experimental studies designed to prevent health problems by assigning interventions to individuals in the participants’ communities (such as a city or state).
    3. Field trial: Studies carried out “on the ground” or “in the field” are conducted on people who are not enrolled in an institution, but are randomly selected from the population. We examine this here in order to determine whether a particular intervention reduces the likelihood of developing a disease.


  • Directly calculating incidence is possible.
  • Probability of occurrence (RR) measured directly.
  • The risk factor can be studied for more than one outcome.
  • Exposure can be studied in relation to dose-response.
  • It is possible to see the temporal relationship between exposure and outcome.
  • Biases like recall bias and interviewer bias don’t pose a problem.


  • The major disadvantage is the high demand for resources, such as personnel, money, and time.
  • Rare diseases are ineligible.
  • A long-term follow-up is necessary.
  • The long follow-up required results in attrition.
  • As the evidence of the RF accumulates, the investigator becomes responsible for educating those with the risk factor. It is unethical to watch and wait.
  • A few or only one risk factor can be studied.
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