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.
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.
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.
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
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:
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)
Risk Factor ⊖
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.
RR: Used by Clinicians
AR: Used by Epidemiologist
PAR: Used by Public Health Program Manager
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