Types of epidemiology

Types Of Epidemiology

The epidemiological studies are broadly divided into two types. The first is the observational studies and second is experimental studies.

1. Observational studies

In observational studies we do not interfere in the process. Observational studies evaluate the disease and associated factors without having any active control on the patient’s actual conditions and exposures.

Observational studies are divided into two types descriptive studies and analytical studies. Furthermore, analytical studies are divided into paste control and quote studies.

What are the descriptive studies?

In descriptive study the disease distribution is studied in a population and described in terms of time place and person.

This gives clues regarding the risk factors for the disease one or more which may actually be the cause of the disease.

A Case report is also a type of descriptive study of a single individual with the disease. Similarly, case series are also a study of small group of cases of the disease. These are useful when the disease is very rare and causes exclusively by a unique type of exposure. For example:

  • Smoking and lung cancer
  • Polyandry and cervical cancer
  • Chloride and angiosarcoma
  • diethylstilbestrol (DES) and clear cell carcinoma of vagina

Best example of descriptive study: Cholera epidemic in London

Cholera epidemic in London:

Cholera emerged as an epidemic in London in 1800s. At that time the bacteria causing cholera was unknown. There was no microscope. So, to search the cases in population a descriptive study was launched. The descriptive study showed that the prevalence of Cholera was more around a particular hand-pump. This provided a clue that was infectious and transmitted through water. So, people pulled out handle of that hand pumps without knowing the causative bacteria. This leaded to end of the epidemic.

What are the Analytical observational studies?

These are the second type of observational studies, which mainly includes two types of studies: Case control and cohort studies. Analytical observational studies also include cross-sectional and ecological studies. Here, we will discuss the most frequently used observational analytical studies mentioned as below:

Case control study:

A set of cases is assembled with a particular disease. Similarly, set of control is chosen which includes people without that disease. The people under control group is chosen after a meticulous matching with the people under case group regarding various baseline and demographic characteristics. The matching is done mainly in terms of age, gender and socio-economic factors.

With the help of an analytical study, we try to analyze history of exposure to the identified risk factors. These risk factors are ascertained from all the members of both the groups.

The history of exposure is confirmed with the help of questionnaire and the previous records. The prevalence of exposure in cases is compared with the prevalence exposure in control group.

Factors found in significantly higher prevalence in diseased group are affirmed as a risk factors of that disease. Similarly, the factors which are significantly higher in the control are noted as possible protective factors.

In case control study the associated risk is evaluated in terms of odds ratio.

What is odd’s ratio?

Odds of any event is defined as the likelihood of occurrence of an event, expressed as a proportion of the probability that the event will not occur. Therefore, odd’s ration is defined as the ratio of odd in cases verses control group.

Let’s assume, exposed cohort contains A + B members in which A developed disease and B didn’t. Non exposed group includes C + D members in which C developed the disease and D did not.

 Risk factors presentRisk factors absent 
CasesABA+B
ControlsCDC+D

Odd’s Ratio: (A x D)/(C x B)

Cohort study design:

These are another type of analytical studies.

In a cohort studies two groups of people are included.

1. Exposed group

2. Non-exposed group

While doing a cohort study it is made sure that none of the members in both the groups have the disease under investigation. Anyone is found with the disease should be excluded.

Both the cohorts followed up at definite time, at regular intervals, for the development of the disease for the.

The incidence of disease what is compared with incidence of the control cohort word. If the incidence of disease in exposed cohort is significantly higher than that in control cohort, the association is confirmed.

In a cohort study significance of associated risk factors in Expo and loan expert group is evaluated in terms of relative risk.

What is relative risk?

Relative risk is defined as the ratio of incidence among divided by incidence among non-exposed group.

For example:

Let’s assume, exposed cohort contains A + B members in which A developed disease and B didn’t. Non exposed group includes C + D members in which C developed the disease and D did not.

 Disease presentDisease absent 
CasesABA+B
ControlsCDC+D

What is the incidence of disease in exposed group?

=A/ (A + B)

What is the incidence of disease in exposed group?

=C/ (C + D)

Relative risk= [A/ (A + B)]/[C/ (C + D)]

Till now we have discussed different type of observational studies. Now coming to the experimental study.

2. Experimental studies

In experimental studies we evaluate interventions and observe their effect.

Experimental studies are of three types:

A. Randomized control trials: This uses patients as unit of study. These are mostly used to assess a new therapy.

B. Field trials: These types of trials use healthy individuals as units of study. These are mostly used to assess preventive agents. These include vaccine assessment and prophylactic efficacy of an intervention.

C. Community trials: These trials use communities as unit of study. In such trial interventions are evaluated on communities’ level. Interventions are given to few communities and the effect are observed with reference to the other communities.

What are the experimental studies?

As we discussed determinantal studies are carried to make a hypothesis. Hypothesis are there with the help of observational studies including case control and the cohort study. However own findings are finally conferred with the help of experimental study.

Experimental studies are mostly conducted for assessment of rabies vaccines, or risk factors. Experimental studies have lot of ethical issues. In an experimental study, cases of disease are identified. These cases are further included in the study after the informed consent.

1. Randomized control studies

Randomized controlled study eligible patients informed consent are randomly allocated to different types of study groups.

For example: Let’s consider placebo-controlled study.

In such studies one group of presents are treated with and experimental intervention and another are provided with simply a placebo.

In other words, we can say that the study group is deliberately exposed to the therapy while the other is not. Except the given therapy, all patients are kept under identical conditions. All these patients are regularly followed for the percentage of cure rate or incidence of an event.

If the cure rate is significantly high that is considered as better. It confirms the association of an event under observation.

While talking about an experimental study, we must consider the blinding status of that study. Blinding is a very important aspect of experimental studies. Blinding is a very important step to avoid any study bias.

Types of Blinding

Blinding of a study can be done in three ways:

1. Single blinding: When patient does not know whether he or she is getting the drug or placebo

2. Double blinding: When the patient and the dispensing doctor both do not know whether the present in getting the drug or placebo

3. Triple blinding: When the patient, investigator/dispensing doctor and the data analyzers do not know the identity of groups as a therapy or placebo

In all these blinding approaches, the triple blinding is considered as the best.

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