**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, 9Median =

__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

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**

__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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