Principles of Probability
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“Probability is the very guide of life”
-Cicero
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What is Probability?
• The likelihood of an
event occurring.
• Scientist use
probability to
predict random
behavior.
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Probability of an event
The probability P is:
P =
actual outcomes
total number of possible outcomes
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• Probabilities are between 0 and 1.
• Sum of probabilities is always 1.
• If an event probability = 1, it always happens.
• If an event probability = 0, it never happens.
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Calculating Probability
A committee has 12 students:
• 3 seniors
• 5 juniors
• 4 sophomores
The probability a student randomly selected
will be a senior is:
P = 3 seniors ÷ 12 students = 0.25
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Law of Large Numbers
The more trials, the better the odds the
relative frequency of an event gets closer
to the true probability.
• Certainty from uncertainty
• Order from chaos.
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