Chapter 16 Events and Probability Spaces688
Rule 16.5.4(Union Bound).
PrŒE 1 [[En[çPrŒE 1 çCCPrŒEnçC: (16.6)
The Union Bound is useful in many calculations. For example, suppose thatEiis
the event that thei-th critical component amongncomponents in a spacecraft fails.
ThenE 1 [[Enis the event thatsomecritical component fails. If
Pn
iD 1 PrŒEiç
is small, then the Union Bound can provide a reassuringly small upper bound on
this overall probability of critical failure.
16.5.3 Uniform Probability Spaces
Definition 16.5.5.A finite probability space,S, is said to beuniformif PrŒ!çis the
same for every outcome! 2 S.
As we saw in the strange dice problem, uniform sample spaces are particularly
easy to work with. That’s because for any eventES,
PrŒEçD
jEj
jSj: (16.7)
This means that once we know the cardinality ofEandS, we can immediately
obtain PrŒEç. That’s great news because we developed lots of tools for computing
the cardinality of a set in Part III.
For example, suppose that you select five cards at random from a standard deck
of 52 cards. What is the probability of having a full house? Normally, this question
would take some effort to answer. But from the analysis in Section 14.7.2, we know
that
jSjD52
5
!
and
jEjD 13 4
3
!
12
4
2
!
whereEis the event that we have a full house. Since every five-card hand is equally
likely, we can apply equation (16.7) to find that
PrŒEçD13 12
4
34
252
5