Exam Oriented Epidemiology – At A Glance

Epidemiology is the branch of medicine which deals with the incidence, distribution, and possible control of diseases and other factors relating to health. 

This article will provide only the basic knowledge of epidemiology, specially made to answer the questions that appear in ERPM and PLAB exams.


Terminology
  • Mean : The average of the group of values
  • Mode : The value that occurs most frequently
  • Median : Middle value when arranged in order
  • Sensitivity : The proportion of true positives out of those who who have the disease
  • Specificity : The proportion of the true negatives out of those who do not have the disease
  • Prevalence : The proportion of individuals in a population having a disease
  • Incidence : Number of new cases divided by the total population per year who are at risk of becoming a case/diseased
  • Absolute risk : It is the risk of an individual of risk of developing the disease over a time period
  • Relative risk : Used to compare the risk in two different groups of people
  • Odd ratio : Measure of association between exposure and an outcome
 
 
Mean, Mode, Median
 
There are 5 students in the class. Marks for the science paper is given below.
5, 9, 7, 5, 4
Find the mean, mode and median
 
First arrange in order – 4, 5, 5, 7, 9
 
Mean = Sum of all terms = 4 + 5 + 5 + 7 + 9 = 6
              Number of terms               5
 
Mode = 5 (Most frequently repeated number)
 
Median = 5 (Middle valve, when arranged in order)
 
If the order was as 4, 5, 5, 7, 8, 9
Median = 5 + 7= 8.5
                   2
 
 
Sensitivity And Specificity
 
100 individuals (Total population) did the XY test and the results are as follows….
Total population = 100
XY test positive = 65 (Out of them 50 was truly diseased)
XY test negative = 35 (Out of them 5 was truly diseased)
Find the sensitivity and specificity
Find the prevalence
Find the predictive values
 
First arrange these values in the chart as show below.


Total participants = 100

True positives (A) = 50
False positives (B) = 15
False negatives (C) = 5
True negatives (D) = 30
 
Sensitivity =     A        50    = 50 = 0.9
                      A + C     50 + 5    55
 
Specificity =     D       30     = 30= 0.67
                       B + D   15 + 30    45
 
Prevalence =           A + C      x 100%  =   50 + 5  x 100% = 55%
                     A + B + C + D                         100
 
Positive predictive value : The probability that subjects with a positive screening test truly have the disease.

Negative predictive value : The probability that subjects with a negative screening test truly don’t have the disease
 
Positive predictive value =     A    x 100 = 50 x 100 = 76.9%
                                            A + B              65
 
 
Negative predictive value =     D    x 100 = 30  x 100 = 85.7%
                                             C + D               35
 
Incidence                Number of cases           
                    Population in a given time period
 
* Prevalence does not have a time period
 
 
Absolute Risk, Relative Risk And Odd Ratio
 
In the city of A, absolute risk of getting W disease is 4 in 100 non smokers.
Smoking increases the risk by 50 %
Calculate the new absolute risk
 
In these type of questions, draw a box as below
In this question there is a 50% increase of the risk upon exposure
 
Absolute risk =  4    = 0.04 or 4%
                         100
 
Upon exposure, there is a 50% increase of the disease.
After exposure, number of diseased = (50 x 4) + 4 = (2 + 4) = 6
                                                                100
 
New absolute risk =   6    = 6%
                                 100

Relative risk = A / (A + B) =  6  = 1.5
                         C / (C + D)     4
 
If the relative risk is more than 1 – Exposure is a risk factor
If the relative risk is less than 1 – Exposure is a protective factor
 
Odd ratio = A/B = AD
                    C/D    BC
 
If the odd ratio is more than 1 – Exposure is a risk factor
If the odd ratio is less than 1 – Exposure is a protective factor
 
 
Reducing Bias / Blinding
  • Single blind – Patient doesn’t know about the medicine that she/he is taking
  • Double blind – Patient and the researcher doesn’t know about the medications used
  • Triple blind – Patient, researcher and the monitoring body doesn’t know about the medications used


Study Design

Study design


Difference Between Case Control Studies And Cohort Studies

Case Control Studies
Cohort Studies
Retrospective study Prospective study
Find a set of people with a disease to find a risk factor Find a set of people with the same risk factors to see what happens in the future
Cheaper Expensive
Easier to conduct Hard, time consuming, sample drop outs
Good for rare diseases



Randomized Control Trails

It is a study conducted to test out a drug or a new treatment modality where two similar group of people are assigned with the testing drug and a placebo. Both of these groups are followed up and recorded.


Standard Deviation

Shows gaussian distribution / bell shape curve


Normal distributions should have the following features….

  • Symmetric bell shape 
  • Mean and median are equal and both are located at the center of the distribution 
  • 68.2% – Falls within the first standard deviation 
  • 95.4% – Falls within the second standard deviation  
  • 99.7% – Falls within the third standard deviation  
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