Classification Of Epidemiological Studies

Classification of Epidemiological Studies is considered an important topic for NEET PG exam as it provides a fundamental understanding of the design and purpose of different types of epidemiological studies. Epidemiology is a branch of medicine that deals with the study of the distribution and determinants of health and disease in populations. 

A solid understanding of the different study designs used in epidemiology is crucial for medical professionals, especially in the field of preventive and social medicine, as it allows them to critically evaluate the results of studies and make informed decisions regarding public health policies and interventions.

Read this blog to understand the classification of epidemiological studies for NEET PG/NExT exam preparations.

Epidemiological Studies

Epidemiological studies mainly deal with the identification of causes/risk factors of a disease so that it can be prevented or controlled. For this purpose, a HYPOTHESIS is generated first. 

Hypothesis

It is a hypothetical statement. After creating a Hypothesis, Epidemiological studies are used to study the made hypothesis. There are 2 types of epidemiological studies: 

Among this, the COHORT and CASE-CONTROL studies are the most important from an exam point of view.

COHORT STUDY

Exposed	Not Exposed
Disease	No Disease	Disease	No Disease

• Start with exposure and go towards the outcome. Has 2 groups: exposed/ not exposed 
• Aka forward looking study/ prospective study. Also called as Cause to Effect study / Risk factor to Disease study / Exposure to outcome study
• Example: Smoking leads to lung cancer
• Keep following up of exposed and non-exposed group and then comparison is made with the no. of new cases between two groups,. hence it is also known as Follow up and incidence study
• Disadvantage: Time consuming 

CASE CONTROL STUDY 

• Participants are classified into 2 groups having disease/ not having disease. Opposite to cohort study. Starting with the disease and going in backward direction
• Study is from Disease to Risk factor, Hence called Retrospective Study/Backward looking study 
• Starting with effect and going towards the causative agent 
• Aka effect to cause study 

• Also called Outcome to exposure study/Disease to Risk factor study. Example: No lung cancer patients had exposure to smoking in the past. It’s a very fast study because no follow up, no new cases 
• Aka case Reference Study/ TROHOC Study. 
• Its opposite in direction of Cohort study
• COHORT means group of individuals having similar characteristics

Important Information

• For cohort study minimum 2 groups are required to do a very good cohort study.
• Case control study also has 2 groups but ideal cases: control ratio is 1:4

STRENGTH OF ASSOCIATION 

We use the strength of association between the disease and causative factor to calculate the risk in the COHORT study. There are 3 relative measures to calculate the strength of association. They are:                

• RR
• AR
• PAR

Relative risk (RR)

• Explains how strongly cause and effect are associated with each other 
  • Relative Risk =(Incidence in Exposed) / (Incidence in Non Exposed)

• Let’s take a COHORT study establishing the connection between smoking & lung cancer. In this case, if RR = (Incidence of lung cancer in smokers) / (Incidence of lung cancer in non-smokers) = 8, it implies that smokers have an 8 times higher chance of getting lung cancer than non smokers.
• Relative risk is also called the risk ratio, ratio of lung cancer between exposed and non-exposed is 8:1. It is mainly used by clinicians. RR is more accurate, M/c used and easiest to calculate the strength of association

Attribute Risk (AR)

• How much disease is attributed to the risk factor in the study? 
  • (Incidence in Exposed-Incidence in Non Exposed) / (Incidence in Exposed) = 100
• Example: If AR = 88%, it means that 88% of lung cancer is attributed to smoking.. Excess risk, absolute risk and risk difference are some synonyms of attribute risk

Population Attributable Risk (PAR)

• Helps to decide whether to start a new health program or not 
  • PAR = (Incidence Total-Incidence in Non Exposed) / (Incidence in Total) = 100
• Example: If PAR=77%, it means that if smoking risk factor is modified or eliminated then there will be an annual reduction of new lung cancer cases every year by 77%

Interpretation of RR

• RR / OR > 1: Association is present between risk factor disease
• RR / OR = 1: No Association between risk factor and disease 
• RR / OR <1: Inverse Association or negative i.e., Risk factor is protective for disease.

Odds ratio (OR) (Case Control study)

	Disease
H/o ⊕	A	B
Risk Factor ⊖	C	D

• Relative Risk is More Accurate than Odds Ratio as Recall Bias can occur in Odds ratio
• OR = AD / BC (Aka cross product ratio)
• OR > 1 : Risk factor

     = 1 : No association

     < 1 : No risk factor

• RR is found in cohort study
• OR is found in case control study
• OR is faster to calculate but RR is better to calculate
• RR is also Most Accurate Measure of Strength of Association in a Cohort Study, Most Easy to Calculate also.

Important Information

• RR: Used by Clinicians
• AR: Used by Epidemiologist
• PAR: Used by Public Health Program Manager

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