# 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.

References:
1. Park K. Health information and basic medical statistics. In: Park’s Textbook of Preventive and Social