Epidemiological methods of studies

By Abdulla Israr & Simran Rani

 EPIDEMIOLOGICAL METHODS OF STUDIES 

# For better experience use your desktop and description also available in video format (attached below)

Introduction

Epidemiological studies involves the monitoring of prevalence of the disease, identifying the natural history and mark the causes of the disease for the prevention and control of the disease. The epidemiological methods have a very broad classification which is as follows-

1) Descriptive

    a) Population

  • Correlational Studies
    b) Individuals
  •  Cross Sectional Surveys
  •  Case Reports
  •  Case Series
2) Analytical 

    a) Observational
  • Case Control Studies
  • Cohort Studies
    b) Experimental
  • Clinical Trials
  • Community trials

1) Descriptive Epidemiology

The descriptive epidemiology helps to describe the general characteristics of a disease in relation to the Time (when), Place (where) & Person (who).

a) Case Report
  • In this study the investigator formulate a hypothesis on the basis of a Case Report that is published in medical journals.
  • For an example, A child is diagnosed with Intestinal obstruction and he also took a shot of Rota virus vaccine three months ago and this case report was published in a medical journal. After reading this medical journal the Investigator formulated a hypothesis which claims that Rota virus vaccine may responsible for the occurrence of this event. 
  • The limitation of this study is the presence of any exposure may be coincidental.
b) Case Series
  • In this study the investigator formulate a hypothesis on the basis of a series/number of cases of the same disease at the same period of time.
  • For an example, Eight cases of lungs cancer were admitted in hospital during the same time period, After taking history of all patients it was founded out that they all were coal miners and in the pathological testing all 8 patients showed the high concentration of Radon gas, then the investigator formulated a hypothesis that the lungs cancer is related to the exposure to Radon gas.
  • The limitation of this study is the lack of comparison group.
c) Correlation Study
  • It can be defined as a study that shows the frequency of a disease by comparing two quantitative variables.
  • For an example, Countries having highest average temperature have the highest rates of CVS (Cerebrovascular Stroke), hence the mentioned statement shows a (+) correlation between the temperature and CVS.
  • The limitation of this study is the absence of Confounding factor (factors other than the studied one that also contribute in development of the disease)
d) Cross Sectional Study
  • This study helps us to calculate the Prevalence Rate of a disease by dividing the population into four different sections.
      • With Exposure and With Disease
      • With Exposure and Without Disease
      • Without Exposure and With Disease
      • Without Exposure and Without Disease
           After forming the 4 sections the epidemiologist calculate the prevalence rate of disease by                    =   Total number of cases at a time x 100
                        Total number of population at the same time
  • For an example, An investigatory wants to calculate the prevalence rate of Obesity among 400 Secondary school child in which he considered out the High Caloric Diet as an exposure to Obesity. So, lets forms out the 4 section
    • Section 1 - Children having high caloric diet consumption & with Obesity = 20
    • Section 2 - Children having high caloric diet consumption & without Obesity = 60
    • Section 3 - Children not having high caloric diet consumption & with Obesity = 16
    • Section 4 - Children not having high caloric diet consumption without & Obesity = 304
  • The above numbers of four different sections can also be distributed as
    • Consumption of high caloric diet     With Obesity     Without Obesity     Total
                                    YES                                            20                        60                  80
                                     NO                                             16                       304                320

                                    Total                                           36                       364                400
  • Now calculate the Prevalence Rate
    • Prevalence rate of obesity among those consumed high caloric diet=(20 X 100)/80 = 25%
    • Prevalence rate of obesity among those don't consume high caloric diet=(16 X 100)/320= 5%
  • The limitation of this disease is that it deals only with survivors but not those who died, cured or migrated.

2) Analytical Epidemiology

The analytical epidemiology helps to identify the relationship between risk factors and occurrence of disease.
                   
a) Case Control    

  • Case Control study is a backward phenomenon that aims to calculate out the risk associated with exposure. In this study the subjects are selected and divided into two categories
    • Cases (Subjects having disease with or without exposure)
    • Control (Subjects free from disease with or without exposure)
  • After that the assessment, analysis & interpretation of the exposure has been done.
  • The steps involved in the Case Control study are-
    • Selection of Cases (from hospitals & general population)
    • Selection of Control (relatives & neighborhood and must have same confounding factors as cases) 
    • Assessment of the exposure rate
    • Analysis and interpretation of odds ratio
  • Let's understand the whole concept by the help of an example of Case-Control study of  Lungs Cancer.
a) Selection of Cases (Patients having lungs cancer)                                      

The cases of lungs cancer can be categorized into two different categories
  • Were Exposed to cigarette smoking = 40 
  • Were not Exposed to cigarette smoking = 20
b) Selection of Control (Peoples free from lungs cancer)                                     

The controls of the study can also be categorized into two different categories
  • Were Exposed to cigarette smoking = 20 
  • Were not Exposed to cigarette smoking = 120
c) Assessment of the Exposure

The assessment of the exposure can be done by the help of
  • Questionnaires
  • Interview
  • Hospital records
  • Occupational records etc.
d) Analysis & Interpretation of the results

This last step of Case Control study involves the 3 basic & calculative steps-
  • Tabulation of the data
    • Cigarette Smoking            Cases     Control     
      •              YES                           40             20
                    NO                            20            120                        

                  Total                           60            140

  • Exposure Rate
    • Among Cases
                                =  Number of Cases having exposure to cigarette X 100 = (40/60)*100 = 66%
Total number of Cases

    • Among Controls 
                            =  Number of Controls having exposure to cigarette X 100 = (20/140)*100 = 14%
Total number of Cases

  • Odds Ratio (Estimated Risk)
    1. What is the odds that a case is being exposed = a/c (40/20)
    2. What is the odds that a control is being exposed = b/d (20/120)
    3. What is the estimated risk (Odds Ratio) = ad/bc (4800/400) = 12:1

  • The limitation of this study is that it is not suitable for studying the rare exposure and have the problem of bias (systemic error).

b) Cohort (Prospective study)

  • Cohort study is a forwarded phenomenon that focuses to identify the outcomes of any exposure in future, the word Cohort means a group of people who shares a common characteristics in a defined period of time.
  • A group of individuals are defined on the basis of presence or absence of exposure to any risk factor and at the time when the exposure status is defined, all individuals must be free from disease under investigation.
  • They will be followed over a period of time to assess the occurrence of that outcome and the steps involved in Cohort are same as Case Control but the direction of Cohort study is from Present to Future.
  • Lets understand this study by the help of an example -
    •  Two cohorts are classified according to the exposure to sunrays
      • Exposed to sunrays (n = 400)
      • Not exposed to sunrays (n = 400)
    • These two cohorts are similar in age, sex and social class.
    • They are followed up for ten year period and among those-
      • Exposed to sunrays = 40 BCC (Basal Cell Cancer) cases found
      • Not exposed to sunrays = 4 BCC cases found
    • Tabulation of the data
      • Sunrays Exposure           Persons with BCC    Persons without BCC    Total
        •              YES                           40                                     360                          400
                      NO                            4                                       396                          400

    • Calculation of the rate of occurrence of BCC
      • Incidence of BCC among exposed (Ie) = (40X100)/400 = 10%
      • Incidence of BCC among not exposed (Io) = (4X100)/400 = 1%
    • Calculation of Risk
      • Relative Risk Ratio (RR) = Ie/Io = 10/1 = 10:1
      • Attributable Risk Percent (ARP) = (Ie-I0/Ie) * 100 = 90%
    • Interpretation
      • 90% of the BCC is attributed to exposure to sunrays that means 90% of BCC could be prevented if persons avoid exposure to sunrays

3) Clinical Trials
  • The clinical trials used to determine the efficacy of a new line drug or to compare the two types of drugs.
  • Diseased subjects randomly divided into two groups
    • Group having the dose of new drug
    • Group having usual placebo
  • Results are assessed by comparing health improvement of the above two groups at the end of the trial.
4) Community Trials
  • It involve persons who are not diseased but presumed likely to be at risk and the sample is drawn from the community.
  • Data collection takes place in the field.
  • For an example, field trial of Covid-19 vaccine in India.

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Comments

Saqlain Gulzar said…
Good work and keep it up
February 6, 2023 at 5:13 PM
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