Page 17 - Probability Demystified

6 CHAPTER 1 Basic Concepts

probability of the two outcomes when flipping a coin. When the probability
of an event is close to one, the event is almost sure to occur. For example,
if the chance of it snowing tomorrow is 90%, more than likely, you’ll see
some snow. See Figure 1-1.

Fig. 1-1.

Rule 2: When an event cannot occur, the probability will be zero.

EXAMPLE: A die is rolled; find the probability of getting a 7.

SOLUTION:

Since the sample space is 1, 2, 3, 4, 5, and 6, and there is no way to get a 7,
P(7) ¼ 0. The event in this case has no outcomes when the sample space is
considered.

Rule 3: When an event is certain to occur, the probability is 1.

EXAMPLE: A die is rolled; find the probability of getting a number less
than 7.

SOLUTION:
6
Since all outcomes in the sample space are less than 7, the probability is ¼1.
6

Rule 4: The sum of the probabilities of all of the outcomes in the sample space
is 1.

Referring to the sample space for tossing two coins (HH, HT, TH, TT), each
1
outcome has a probability of and the sum of the probabilities of all of the
4
outcomes is
1 1 1 1 4
þ þ þ ¼ ¼ 1:
4 4 4 4 4