Probability axioms

The assumptions as to setting up the axioms can be summarised as follows: Let ${\displaystyle (\Omega ,F,P)}$  be a measure space with ${\displaystyle P(E)}$  being the probability of some event ${\displaystyle E}$ , and ${\displaystyle P(\Omega )=1}$ . Then ${\displaystyle (\Omega ,F,P)}$  is a probability space, with sample space ${\displaystyle \Omega }$ , event space ${\displaystyle F}$  and probability measure ${\displaystyle P}$ .^[1]

The probability of an event is a non-negative real number:

${\displaystyle P(E)\in \mathbb {R} ,P(E)\geq 0\qquad \forall E\in F}$ 

where ${\displaystyle F}$  is the event space. It follows (when combined with the second axiom) that ${\displaystyle P(E)}$  is always finite, in contrast with more general measure theory. Theories which assign negative probability relax the first axiom.

This is the assumption of unit measure: that the probability that at least one of the elementary events in the entire sample space will occur is 1.

${\displaystyle P(\Omega )=1}$ 

This is the assumption of σ-additivity:

Any countable sequence of disjoint sets (synonymous with mutually exclusive events) ${\displaystyle E_{1},E_{2},\ldots }$  satisfies

${\displaystyle P\left(\bigcup _{i=1}^{\infty }E_{i}\right)=\sum _{i=1}^{\infty }P(E_{i}).}$ 

Some authors consider merely finitely additive probability spaces, in which case one just needs an algebra of sets, rather than a σ-algebra.^[5] Quasiprobability distributions in general relax the third axiom.

From the Kolmogorov axioms, one can deduce other useful rules for studying probabilities. The proofs^[6][7][8] of these rules are a very insightful procedure that illustrates the power of the third axiom, and its interaction with the prior two axioms. Four of the immediate corollaries and their proofs are shown below:

${\displaystyle \quad {\text{if}}\quad A\subseteq B\quad {\text{then}}\quad P(A)\leq P(B).}$ 

If A is a subset of, or equal to B, then the probability of A is less than, or equal to the probability of B.

In order to verify the monotonicity property, we set ${\displaystyle E_{1}=A}$  and ${\displaystyle E_{2}=B\setminus A}$ , where ${\displaystyle A\subseteq B}$  and ${\displaystyle E_{i}=\varnothing }$  for ${\displaystyle i\geq 3}$ . From the properties of the empty set (${\displaystyle \varnothing }$ ), it is easy to see that the sets ${\displaystyle E_{i}}$  are pairwise disjoint and ${\displaystyle E_{1}\cup E_{2}\cup \cdots =B}$ . Hence, we obtain from the third axiom that

${\displaystyle P(A)+P(B\setminus A)+\sum _{i=3}^{\infty }P(E_{i})=P(B).}$ 

Since, by the first axiom, the left-hand side of this equation is a series of non-negative numbers, and since it converges to ${\displaystyle P(B)}$  which is finite, we obtain both ${\displaystyle P(A)\leq P(B)}$  and ${\displaystyle P(\varnothing )=0}$ .

${\displaystyle P(\varnothing )=0.}$ 

In many cases, ${\displaystyle \varnothing }$  is not the only event with probability 0.

${\displaystyle P(\varnothing \cup \varnothing )=P(\varnothing )}$  since ${\displaystyle \varnothing \cup \varnothing =\varnothing }$ ,

${\displaystyle P(\varnothing )+P(\varnothing )=P(\varnothing )}$  by applying the third axiom to the left-hand side (note ${\displaystyle \varnothing }$  is disjoint with itself), and so

${\displaystyle P(\varnothing )=0}$  by subtracting ${\displaystyle P(\varnothing )}$  from each side of the equation.

${\displaystyle P\left(A^{c}\right)=P(\Omega -A)=1-P(A)}$ 

Given ${\displaystyle A}$  and ${\displaystyle A^{c}}$  are mutually exclusive and that ${\displaystyle A\cup A^{c}=\Omega }$ :

${\displaystyle P(A\cup A^{c})=P(A)+P(A^{c})}$  ... (by axiom 3)

and, ${\displaystyle P(A\cup A^{c})=P(\Omega )=1}$  ... (by axiom 2)

${\displaystyle \Rightarrow P(A)+P(A^{c})=1}$ 

${\displaystyle \therefore P(A^{c})=1-P(A)}$ 

It immediately follows from the monotonicity property that

${\displaystyle 0\leq P(E)\leq 1\qquad \forall E\in F.}$ 

Given the complement rule ${\displaystyle P(E^{c})=1-P(E)}$  and axiom 1 ${\displaystyle P(E^{c})\geq 0}$ :

${\displaystyle 1-P(E)\geq 0}$ 

${\displaystyle \Rightarrow 1\geq P(E)}$ 

${\displaystyle \therefore 0\leq P(E)\leq 1}$ 

Another important property is:

${\displaystyle P(A\cup B)=P(A)+P(B)-P(A\cap B).}$ 

This is called the addition law of probability, or the sum rule. That is, the probability that an event in A or B will happen is the sum of the probability of an event in A and the probability of an event in B, minus the probability of an event that is in both A and B. The proof of this is as follows:

Firstly,

${\displaystyle P(A\cup B)=P(A)+P(B\setminus A)}$ . (by Axiom 3)

So,

${\displaystyle P(A\cup B)=P(A)+P(B\setminus (A\cap B))}$  (by ${\displaystyle B\setminus A=B\setminus (A\cap B)}$ ).

Also,

${\displaystyle P(B)=P(B\setminus (A\cap B))+P(A\cap B)}$ 

and eliminating ${\displaystyle P(B\setminus (A\cap B))}$  from both equations gives us the desired result.

An extension of the addition law to any number of sets is the inclusion–exclusion principle.

Setting B to the complement A^c of A in the addition law gives

${\displaystyle P\left(A^{c}\right)=P(\Omega \setminus A)=1-P(A)}$ 

That is, the probability that any event will not happen (or the event's complement) is 1 minus the probability that it will.

Consider a single coin-toss, and assume that the coin will either land heads (H) or tails (T) (but not both). No assumption is made as to whether the coin is fair or as to whether or not any bias depends on how the coin is tossed.^[9]

We may define:

${\displaystyle \Omega =\{H,T\}}$ 

${\displaystyle F=\{\varnothing ,\{H\},\{T\},\{H,T\}\}}$ 

Kolmogorov's axioms imply that:

${\displaystyle P(\varnothing )=0}$ 

The probability of neither heads nor tails, is 0.

${\displaystyle P(\{H,T\}^{c})=0}$ 

The probability of either heads or tails, is 1.

${\displaystyle P(\{H\})+P(\{T\})=1}$ 

The sum of the probability of heads and the probability of tails, is 1.

• Borel algebra – Class of mathematical setsPages displaying short descriptions of redirect targets
• Conditional probability – Probability of an event occurring, given that another event has already occurred
• Fully probabilistic design
• Intuitive statistics – cognitive phenomenon where organisms use data to make generalizations and predictions about the worldPages displaying wikidata descriptions as a fallback
• Quasiprobability – Concept in statisticsPages displaying short descriptions of redirect targets
• Set theory – Branch of mathematics that studies sets
• σ-algebra – Algebraic structure of set algebraPages displaying short descriptions of redirect targets

• DeGroot, Morris H. (1975). Probability and Statistics. Reading: Addison-Wesley. pp. 12–16. ISBN 0-201-01503-X.
• McCord, James R.; Moroney, Richard M. (1964). "Axiomatic Probability". Introduction to Probability Theory. New York: Macmillan. pp. 13–28.
• Formal definition of probability in the Mizar system, and the list of theorems formally proved about it.