1. Objective- To differentiate between probability and relative frequency and to solve problems involving both.
If a woman were to have a baby in 1990, what is
the probability that it would be a boy?
# of favorable outcomes
Probability =
# of possible outcomes
boy 1
P (boy) = =
= 50 %
boy or girl 2
Probability involves predicting future events.

2. Probability involves predicting future events.
Relative Frequency involves data from past events.
# of times an event occurred
Relative Frequency =
# of times it could have occurred
# of boys born in 1990
2,129,000
r = =
0.512
total # of births in 1990
4,158,000
r %
Based on relative frequency, the probability
of having a boy is actually 51.2%.

3. In 1990, the state of Illinois tested 3840 skunks for rabies, of which 1446 actually had rabies. What was the relative frequency of skunks with rabies?
frequency
1446
r = =
0.377
total opportunities
3840
r %

4. If a hurricane is likely to occur on any day of
the week, what is the probability that it will
occur on a weekend?
# of days in weekend 2
P (hurricane) = =
# of days in week 7 2
or % 7

5. Probability and relative frequency are always
expressed as fractions ( or decimals ) between
0 and 1.
Probability-future
impossible
certain
Relative Frequency-past
never occurred
always occurred

6. + = 1 Complementary Events Two events are complementary if their
intersection is the empty set and their
union is the set of all possible outcomes.
Complementary
P(Hurricane on weekend) P(Hurricane on weekday) +
= 1
The sum of probabilities for complementary events
always equals 1.
P(It will rain) P(It will not rain)
30% % = 100%