1. Turn in AP Registration
Chapter 7 Random Variables a variable whose value is a numerical outcome of a random phenomenon.
7.1 Discrete and Continuous Random Variables
7.2 Means and Variances of Random Variables
26 Discrete & Continuous, Mean/Var, Law of Large #s
7.2, 7.7, 7.17, 7.24, 7.27, 7.28, 7.32 27
Rules for Means and Variances
7.38, 7.47, 7.49, 7.60 28
REVIEW 29 30
CHAPTER 7 TEST
Turn in AP Registration

2. 7.1 Discrete and Continuous Random Variables
Discrete Random Variable
a random variable with a countable number of outcomes
the probability distribution lists the values and their probabilities.
Example: Mrs. Pfeiffer’s Grade Distribution
X = 15% A’s, 30% B’s, 30% C’s, 15% D’s, and 10% F’s
x = Grade
Probability
Find the probability that a randomly selected student receives a B or better. P(x ≥ 3) =

3. 7.1 Discrete and Continuous Random Variables
a random variable that takes all values in an interval of numbers
the probability distribution is described by a density curve
the probability of an event is the area under the density curve
Example: Let S = {all numbers 𝑥 such that 0≤𝑥≤1}
The result of many trials are represented by the density curve of a uniform distribution.
0 1
a. 𝑃(.3≤𝑥≤.7) b. 𝑃(𝑥=.6) c. 𝑃(𝑥>.5)

4. Means of Discrete Random Variables
Mean of a Discrete Random Variable
(aka Expected Value or Weighted Average)
Mean = E(X) = µx = 𝑥 𝑖 𝑝 𝑖
Example – Suppose you are playing the Missouri Pick 3 Lotto. If you match the 3 numbers exactly, you win $500. How much can you expect to win?
On Pink Sheet

5. Expected Value Explanation
We predict that in the LONG RUN, the average Lottery ticket buyer wins an average of $0.50 each time. Look at it from the state’s standpoint: Lotto tickets cost $1, so in the LONG RUN, the state keeps half of the money everyone wages. Expected Value can be misleading – we don’t expect to win $0.50 on one lottery ticket.

6. Variances of Discrete Random Variables
Variance of a Discrete Random Variable
Variance = Var(X) = σx = 𝑥 𝑖 − 𝜇 𝑥 2 𝑝 𝑖
Standard Deviation = σx = sq. root of variance
Example – Suppose you are playing the Missouri Pick 3 Lotto. If you match the 3 numbers exactly, you win $500. What is the variation of the payout?
On Pink Sheet 2

7. Example – Mean and Variance of a Discrete Random Variable
Toss 4 coins and record the number of heads. Create a probability distribution table and find the mean and standard deviation of X. X =

8. Example – Mean of a Continuous Random Variable
Find the value of X for which the area under the curve is ½ on each side.
density curve normal distribution
𝜇 𝑥 =1
𝜇 𝑥 =𝜇

9. Law of Large Numbers
Law of Large Numbers says that for ALL POPULATIONS (not just normal distributions) that as the number of observations drawn increases, the mean of the observed values (the sample mean = 𝑥 ) eventually approaches the mean of the population (𝜇). This is why casinos stay in business!!!

10. WARM UP
The distribution of weights of 9 ounce bags of a particular brand of potato chips is approximately Normal with mean μ = 9.12 ounces and standard deviation σ = 0.15 ounce.
Draw an accurate sketch of the distribution of potato chip bag weights. (Be sure to label 1, 2, and 3, standard deviations from the mean.)
A bag that weighs 8.87 ounces is at what percentile in this distribution?
What percent of bags weigh between 8.25 ounces and ounces?
The top 10% of all bags weigh at least what?

11. Rules for Means and Variances