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P-Value: Epidemiology – a Beginner’s Guide

In epidemiology, ‘p-value’ stands for ‘Probability – Value’

Value of probability or p-value ranges from 0 (zero) to 1 (one)
• A p-value of 0 means there is no chance of the event happening e.g. getting a seven on a dice
• A p-value of 1 means a 100% chance of the event happening e.g. Sun will rise in the east and set in the west.

Probability of WHAT?
Probability of the ‘Null – Hypothesis’ being true

So, ‘p-value’ is the probability of the ‘Null – Hypothesis’ being true.

Null hypothesis (H0): There is no significant difference between groups or no effect (of a medicine or intervention) or no relationship between two or more variables. (Details of Null Hypotheses)

Hence if p-value is high, it means that there is the Null-Hypothesis is true and there is
• NO difference between the groups or
• NO effect of the treatment under investigation or
• NO correlation between two variables being tested

A LOW p-value implies a low probability of Null-Hypothesis being true and leads to REJECTION of Null-Hypotheses i.e.
• The groups are actually different from each other
• The treatment under investigation HAS an effect
• The variables under study are correlated

What is the cut-off for high and low p-value?
By convention, the cut-off of the p-value is 0.05.

If p ≥ 0.05, Null-Hypothesis is true and accepted. There is no difference of effect of intervention and no association between variables
If p < 0.05, Null Hypothesis is rejected and any observed difference or association is taken to be a REAL one.

1. Park K. Health information and basic medical statistics. In: Park’s Textbook of Preventive and Social
Medicine, 27th ed. Jabalpur, Madhya Pradesh: M/S Banarasidas Bhanot Publishers; 2023.
2. Patrikar SR. Statistics. In: Vaidya R, Tilak R, Gupta R, Kunte R, editors. Text Book of Public Health and Community Medicine, 1st ed. Pune, Dept. Community Medicine, AFMC, in collaboration with WHO, India office, Delhi; 2009.
3. Tiwari P, Tiwari S. Tests of significance. In: Epidemiology Made Easy. New Delhi: Jaypee Publishers;

What is 'p-value':
video on p-value:
Video in HINDI on p-value:
What is 'Null - Hypothesis'?
video on Null Hypothesis:
HINDI video on Null Hypothesis:
What is 'Confidence Interval'?: