Elementary event

In probability theory, an elementary event, also called an atomic event or sample point, is an event which contains only a single outcome in the sample space.[1] Using set theory terminology, an elementary event is a singleton. Elementary events and their corresponding outcomes are often written interchangeably for simplicity, as such an event corresponding to precisely one outcome.

Part of a series on statistics
Probability theory
Probability Axioms Determinism System Indeterminism Randomness
Probability space Sample space Event Collectively exhaustive events Elementary event Mutual exclusivity Outcome Singleton Experiment Bernoulli trial Probability distribution Bernoulli distribution Binomial distribution Exponential distribution Normal distribution Pareto distribution Poisson distribution Probability measure Random variable Bernoulli process Continuous or discrete Expected value Markov chain Observed value Random walk Stochastic process
Complementary event Joint probability Marginal probability Conditional probability
Independence Conditional independence Law of total probability Law of large numbers Bayes' theorem Boole's inequality
Venn diagram Tree diagram

The following are examples of elementary events:

• All sets ${\displaystyle \{k\},}$ where ${\displaystyle k\in \mathbb {N} }$ if objects are being counted and the sample space is ${\displaystyle S=\{1,2,3,\ldots \}}$ (the natural numbers).
• ${\displaystyle \{HH\},\{HT\},\{TH\},{\text{ and }}\{TT\}}$ if a coin is tossed twice. ${\displaystyle S=\{HH,HT,TH,TT\}}$ where ${\displaystyle H}$ stands for heads and ${\displaystyle T}$ for tails.
• All sets ${\displaystyle \{x\},}$ where ${\displaystyle x}$ is a real number. Here ${\displaystyle X}$ is a random variable with a normal distribution and ${\displaystyle S=(-\infty ,+\infty ).}$ This example shows that, because the probability of each elementary event is zero, the probabilities assigned to elementary events do not determine a continuous probability distribution.