Discrete Uniform Distribution gives rise to Probability Measure
Theorem
Let $\EE$ be an experiment.
Let the probability space $\struct {\Omega, \Sigma, \Pr}$ be defined as:
- $\Omega = \set {\omega_1, \omega_2, \ldots, \omega_n}$
- $\Sigma = \powerset \Omega$
- $\forall A \in \Sigma: \map \Pr A = \dfrac 1 n \card A$
where:
- $\powerset \Omega$ denotes the power set of $\Omega$
- $\card A$ denotes the cardinality of $A$.
Then $\Pr$ is a probability measure on $\struct {\Omega, \Sigma}$.
Proof
From Power Set of Sample Space is Event Space we have that $\Sigma$ is an event space.
$\Box$
We check the axioms defining a probability measure:
\((\text I)\) | $:$ | \(\ds \forall A \in \Sigma:\) | \(\ds \map \Pr A \) | \(\ds \ge \) | \(\ds 0 \) | ||||
\((\text {II})\) | $:$ | \(\ds \map \Pr \Omega \) | \(\ds = \) | \(\ds 1 \) | |||||
\((\text {III})\) | $:$ | \(\ds \forall A \in \Sigma:\) | \(\ds \map \Pr A \) | \(\ds = \) | \(\ds \sum_{\bigcup \set e \mathop = A} \map \Pr {\set e} \) | where $e$ denotes the elementary events of $\EE$ |
Axiom $\text I$ is seen to be satisfied by the observation that the cardinality of a set is never negative.
Hence $\map \Pr A \ge 0$.
$\Box$
Then we have:
\(\ds \map \Pr \Omega\) | \(=\) | \(\ds \dfrac 1 n \card \Omega\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac 1 n \times n\) | Definition of $\Omega$: it has been defined as having $n$ elements | |||||||||||
\(\ds \) | \(=\) | \(\ds 1\) |
Axiom $\text {II}$ is thus seen to be satisfied.
$\Box$
Let $A = \set {\omega_{r_1}, \omega_{r_2}, \ldots, \omega_{r_k} }$ where $k = \card A$.
Then by Union of Set of Singletons:
- $A = \set {\omega_{r_1} } \cup \set {\omega_{r_2} } \cup \cdots \cup \set {\omega_{r_k} }$
Hence:
\(\ds \map \Pr A\) | \(=\) | \(\ds \dfrac 1 n \card A\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac 1 n \card {\set {\omega_{r_1}, \omega_{r_2}, \ldots, \omega_{r_k} } }\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac 1 n \card {\set {\omega_{r_1} } \cup \set {\omega_{r_2} } \cup \cdots \cup \set {\omega_{r_k} } }\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \dfrac 1 n \paren {\underbrace {1 + 1 + \cdots + 1}_{\text {$k$ times} } }\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \underbrace {\dfrac 1 n + \dfrac 1 n + \cdots + \dfrac 1 n}_{\text {$k$ times} }\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \map \Pr {\set {\omega_{r_1} } } + \map \Pr {\set {\omega_{r_2} } } + \cdots + \map \Pr {\set {\omega_{r_k} } }\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \sum_{\bigcup \set e \mathop = A} \map \Pr {\set e}\) |
Hence Axiom $\text {III}$ is thus seen to be satisfied.
$\Box$
All axioms are seen to be satisfied.
Hence the result.
$\blacksquare$
Sources
- 1986: Geoffrey Grimmett and Dominic Welsh: Probability: An Introduction ... (previous) ... (next): $1$: Events and probabilities: $1.3$: Probabilities: Example $11$