Probability Definitions

The term probability has been interpreted in terms of four definitions :

1. Classical Definitions : the classical definition states that if an experiment consists of ’S’ outcomes which are mutually exclusive, exhaustive and equally likely and \(S_A\) of them ate the favourable outcomes of an event A then the probability of the event is

\(P\left( A \right)\; =\; \frac{S_A}{S}\)

In other words the probability of event A is equal to the ratio of the number of favourable outcomes \(S_A\) to the total number of outcomes.

2. Axiomatic Definition : In the axiomatic definition of probability, the probability of outcome A is defined by a number of   assigned to A, such a number satisfies the following axioms :

  1. \(P\left( A \right)\; \geq \; 0\)  i.e.,  \(P(A)\) should be non-negative.
  2. The probability of certain event A = 1. i.e., \(P(A) = 1\) .
  3. If the two events A and B are mutually exclusive then the probability of the event \((A \cup B)\) is 

\(P\left( A\cup B \right)\; =\; P\left( A \right)\; +\; P\left( B \right)\; \)

3. Empirical Definition : In N trails of a random experiment of an event is found to occur m times then the relative frequency of occurrence of the event is \(\frac{m}{N}\) is the limiting value approaches to P. When N increased to infinity then P is called the probability of event A.

i.e., \(P(A) =\lim_{x \to \infty} (\frac{m}{N})\)

4. Subjective Definition : In subjective interpretation of probability the number \(P(A)\) is assigned to a statement which is a measure of out state of knowledge or belief concerning the truth of A. These kinds of probability is worth more often used in our day-to-day life and conversation.


Leave a Reply