Meaning of Probability

The words ‘probably’, ‘doubt’, ‘most probably’, ‘chances’, etc., used in the statements above involve an element of uncertainty.
For example, ‘probably rain’ will mean it may rain or may not rain today.
The uncertainty of ‘probably’ etc can be measured numerically by means of ‘probability’ in many cases.

Use of Probability

Probability started with gambling, it has been used extensively in the fields of Physical Sciences, Commerce, Biological Sciences, Medical Sciences, Weather Forecasting, etc.


A trial is an action which results in one or several outcomes.


An event for an experiment is the collection of some outcomes of the experiment.

Elementary event

An event having only one outcome is called an elementary event. The sum of the probabilities of all the elementary events of an experiment is 1.

Complementary event

For any event E, P (E) + P (E) = 1, where E stands for ‘not E’. E and E are called complementary events.

Note :

1. The probability of a sure event (or certain event) is 1.
2. The probability of an impossible event is 0.
3. The probability of an event E is a number P(E) such that 0 ≤ P (E) ≤ 1.


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