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An outcome \(\; w \in \Omega \;\) is a single result of a random experiment.
An outcome occurs when the result of the random experiment is that outcome.
Outcomes are grouped into events.
Outcomes are usually defined as atomic results: for example, if we throw a die, outcomes are \(\; \{ 1 \}\,, \{ 2 \}\,, \{ 3 \}\,, \{ 4 \}\,, \{ 5 \}\) and \(\; \{ 6 \}\), but not \(\; \{ 1\,, 3 \} \;\) or \(\; \{ 2\,, 5 \} \). Therefore, it seems reasonable to assign same probability to each outcome. However, it is not always easy to define outcomes with equal probability.
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The roll of a die or the toss of a coin can easily be represented with outcomes of equal probability if the toss or the coin are fair.
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The toss of a thumb tack, which can result in the point landing upward or downward, cannot be easily represented with outcomes of equal probability.
1 https://en.wikipedia.org/wiki/Outcome_(probability)#Equally_likely_outcomes