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Winter, 2008
Tuesday, Jan. 15
Stat 321 – Lecture 5
Probability Rules (2.2)
Special case:
In situations where the outcomes of an experiment are
equally likely
to occur (e.g.,
tossing a fair coin), the exact probability of an outcome is 1/
N
, where
N
is the total number of
outcomes possible (p. 57).
If we have a set of outcomes, A, then P(A) = (# of outcomes in A)/
N
.
Example 1:
In 1998 the American Film Institute created a list of the top 100 American films
ever made (http://www.afi.com/tvevents/100years/movies.aspx). Suppose that two people gather
to watch one of these movies and, to avoid potentially endless debates about a selection, decide
to choose a movie at random from the “top 100” list.
The following 2
×
2 table classifies each
movie according to whether it was seen by Allan and whether it was seen by Beth.
The “at
random” selection implies that each of the 100 films is equally likely to be chosen, (i.e., each has
probability 1/100).
Thus, the probabilities of these various events can be calculated by counting
how many of the 100 films comprise the event of interest. Let A denote the event that Allan has
seen the film, so A = {films that Allan has seen} and B denote the event that Beth has seen the
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 Winter '08
 Chance
 Statistics, Probability

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