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Copyright 息 2012 Pearson Canada Inc., Toronto, Ontario
Probability
Conditional Probability
Copyright 息 2012 Pearson Canada Inc., Toronto, Ontario 際際滷 15- 2
Conditional Probability
 When we want the probability of an event from a
conditional distribution, we write P(B|A) and
pronounce it the probability of B given A.
 A probability that takes into account a given
condition is called a conditional probability.
Copyright 息 2012 Pearson Canada Inc., Toronto, Ontario 際際滷 15- 3
Conditional Probability (cont.)
 To find the probability of the event B given the
event A, we restrict our attention to the outcomes
in A. We then find in what fraction of those
outcomes B also occurred.
 Note: P(A) cannot equal 0, since we know that A
has occurred.
(A and B)( | )
( )
PP
P
B A
A
Copyright 息 2012 Pearson Canada Inc., Toronto, Ontario 際際滷 15- 4
Independence
 Independence of two events means that the
outcome of one event does not influence the
probability of the other.
 With our new notation for conditional
probabilities, we can now formalize this definition:
 Events A and B are independent whenever
P(B|A) = P(B). (Equivalently, events A and B
are independent whenever P(A|B) = P(A).)
Copyright 息 2012 Pearson Canada Inc., Toronto, Ontario 際際滷 15- 5
The General Multiplication Rule
 When two events A and B are independent, we
can use the multiplication rule for independent
events :
P(A and B) = P(A) x P(B)
 However, when our events are not independent,
this earlier multiplication rule does not work.
Thus, we need the General Multiplication Rule.
Copyright 息 2012 Pearson Canada Inc., Toronto, Ontario 際際滷 15- 6
The General Multiplication Rule (cont.)
 We encountered the general multiplication rule in
the form of conditional probability.
 Rearranging the equation in the definition for
conditional probability, we get the General
Multiplication Rule:
 For any two events A and B,
P(A and B) = P(A) x P(B|A)
or
P(A and B) = P(B) x P(A|B)
Copyright 息 2012 Pearson Canada Inc., Toronto, Ontario 際際滷 15- 7
Independent  Disjoint
 Disjoint events cannot be independent! Well, why not?
 Since we know that disjoint events have no outcomes
in common, knowing that one occurred means the
other didnt.
 Thus, the probability of the second occurring changed
based on our knowledge that the first occurred.
 It follows, then, that the two events are not
independent.
 A common error is to treat disjoint events as if they were
independent, and apply the Multiplication Rule for
independent eventsdont make that mistake.
Copyright 息 2012 Pearson Canada Inc., Toronto, Ontario 際際滷 15- 8
Depending on Independence
 Its much easier to think about independent
events than to deal with conditional probabilities.
 It seems that most peoples natural intuition for
probabilities breaks down when it comes to
conditional probabilities.
 Dont fall into this trap: whenever you see
probabilities multiplied together, stop and ask
whether you think they are really independent.
Copyright 息 2012 Pearson Canada Inc., Toronto, Ontario 際際滷 15- 9
Tables and Conditional Probability
 It is easy to understand conditional probabilities
with contingency tables

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Conditional probability

  • 1. Copyright 息 2012 Pearson Canada Inc., Toronto, Ontario Probability Conditional Probability
  • 2. Copyright 息 2012 Pearson Canada Inc., Toronto, Ontario 際際滷 15- 2 Conditional Probability When we want the probability of an event from a conditional distribution, we write P(B|A) and pronounce it the probability of B given A. A probability that takes into account a given condition is called a conditional probability.
  • 3. Copyright 息 2012 Pearson Canada Inc., Toronto, Ontario 際際滷 15- 3 Conditional Probability (cont.) To find the probability of the event B given the event A, we restrict our attention to the outcomes in A. We then find in what fraction of those outcomes B also occurred. Note: P(A) cannot equal 0, since we know that A has occurred. (A and B)( | ) ( ) PP P B A A
  • 4. Copyright 息 2012 Pearson Canada Inc., Toronto, Ontario 際際滷 15- 4 Independence Independence of two events means that the outcome of one event does not influence the probability of the other. With our new notation for conditional probabilities, we can now formalize this definition: Events A and B are independent whenever P(B|A) = P(B). (Equivalently, events A and B are independent whenever P(A|B) = P(A).)
  • 5. Copyright 息 2012 Pearson Canada Inc., Toronto, Ontario 際際滷 15- 5 The General Multiplication Rule When two events A and B are independent, we can use the multiplication rule for independent events : P(A and B) = P(A) x P(B) However, when our events are not independent, this earlier multiplication rule does not work. Thus, we need the General Multiplication Rule.
  • 6. Copyright 息 2012 Pearson Canada Inc., Toronto, Ontario 際際滷 15- 6 The General Multiplication Rule (cont.) We encountered the general multiplication rule in the form of conditional probability. Rearranging the equation in the definition for conditional probability, we get the General Multiplication Rule: For any two events A and B, P(A and B) = P(A) x P(B|A) or P(A and B) = P(B) x P(A|B)
  • 7. Copyright 息 2012 Pearson Canada Inc., Toronto, Ontario 際際滷 15- 7 Independent Disjoint Disjoint events cannot be independent! Well, why not? Since we know that disjoint events have no outcomes in common, knowing that one occurred means the other didnt. Thus, the probability of the second occurring changed based on our knowledge that the first occurred. It follows, then, that the two events are not independent. A common error is to treat disjoint events as if they were independent, and apply the Multiplication Rule for independent eventsdont make that mistake.
  • 8. Copyright 息 2012 Pearson Canada Inc., Toronto, Ontario 際際滷 15- 8 Depending on Independence Its much easier to think about independent events than to deal with conditional probabilities. It seems that most peoples natural intuition for probabilities breaks down when it comes to conditional probabilities. Dont fall into this trap: whenever you see probabilities multiplied together, stop and ask whether you think they are really independent.
  • 9. Copyright 息 2012 Pearson Canada Inc., Toronto, Ontario 際際滷 15- 9 Tables and Conditional Probability It is easy to understand conditional probabilities with contingency tables