Notation for Probabilities.

P denotes a Probability
A, B, and C denote specific events
P(A) denotes the probability of event A occuring

1. Relative Frequency Approximation of Probability condcut (or observe) a procedure, and count the numder of times that even A actually occurs. Based on these actual results, P(A) is approximated as follows:



Number of times A occurred
P(A)= -------------------------------------------
number of times the procedure was repeated


2. Classical approach to probability ( requires equally likely outcomes)

Assume that a given porcedure has n different simple events and thet each of those simple events has an equal chance of occuring. If even A can occur in s of these n ways, then:


Number of ways A can occur
P(A)=---------------------------------------------
Number of differen simple events


OR

S
---
N

3.Subjective Probabilities P(A), the probability of an event A, is estimates by using knowledge of the relevant circumstances.

note that the classical approach requires equally likely outcomes. If the oucomes are not equally likely we most use the frequancy approximation or we must rely on our knowledge of the circumstances to make an educated guess.



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