Lectures on Elementary Probability
Lectures on Elementary Probability
Lectures on Elementary Probability
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Chapter 2<br />
<strong>Probability</strong> Axioms<br />
2.1 Logic and sets<br />
In probability there is a set called the sample space S. An element of the sample<br />
space is called an outcome of the experiment.<br />
An event is identified with a subset E of the sample space S. If the experimental<br />
outcome bel<strong>on</strong>gs to the subset, then the event is said to happen.<br />
We can combine events by set theory operati<strong>on</strong>s. Each such operati<strong>on</strong> corresp<strong>on</strong>ds<br />
to a logical operati<strong>on</strong>.<br />
Uni<strong>on</strong> corresp<strong>on</strong>ds to or: s is in E ∪ F if and <strong>on</strong>ly if s is in E or s is in F.<br />
Intersecti<strong>on</strong> corresp<strong>on</strong>ds to and: s is in E ∩ F if and <strong>on</strong>ly if s is in E and s<br />
is in F.<br />
Complement corresp<strong>on</strong>ds to not: s is in E c if and <strong>on</strong>ly if s is in S but is not<br />
in E.<br />
The whole sample space S is the sure event. It corresp<strong>on</strong>ds to true: s is in<br />
S is always true.<br />
The empty set is the impossible event. It corresp<strong>on</strong>ds to false: s is in ∅ is<br />
always false.<br />
Two events E, F are exclusive if E ∩ F = ∅. This means that s in E and s<br />
in F is always false.<br />
2.2 <strong>Probability</strong><br />
The probability rules corresp<strong>on</strong>d to the logical operati<strong>on</strong>s. A probability assignment<br />
P assigns to each event E a number P [E] with 0 ≤ P [E] ≤ 1.<br />
The or rule: If E, F are exclusive, then<br />
The and rule: If E, F are independent, then<br />
P [E ∪ F ] = P [E] + P [F ]. (2.1)<br />
P [E ∩ F ] = P [E]P [F ]. (2.2)<br />
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