Lecture 9
Dustin Lueker


Perfectly symmetric and bell-shaped
Characterized by two parameters
◦ Mean = μ
◦ Standard Deviation = σ

Standard Normal
◦μ=0
◦σ=1
 Solid Line
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
For a normally distributed random variable,
find the following
◦ P(Z>.82) =
◦ P(-.2<Z<2.18) =
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For a normal distribution, how many standard
deviations from the mean is the 90th percentile?
◦ What is the value of z such that 0.90 probability is less
than z?
 P(Z<z) = .90
◦ If 0.9 probability is less than z, then there is 0.4
probability between 0 and z
 Because there is 0.5 probability less than 0
 This is because the entire curve has an area under it of 1,
thus the area under half the curve is 0.5
 z=1.28
 The 90th percentile of a normal distribution is 1.28 standard
deviations above the mean
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
We can also use the table to find z-values for
given probabilities
Find the following
◦ P(Z>a) = .7224
 a=
◦ P(Z<b) = .2090
 b=
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

When values from an arbitrary normal
distribution are converted to z-scores, then
they have a standard normal distribution
The conversion is done by subtracting the
mean μ, and then dividing by the standard
deviation σ
z
x

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
The z-score for a value x of a random variable is
the number of standard deviations that x is
above μ
◦ If x is below μ, then the z-score is negative


The z-score is used to compare values from
different normal distributions
Calculating
◦ Need to know
 x
 μ
 σ
z
x

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SAT Scores
◦ μ=500
◦ σ=100
 SAT score 700 has a z-score of z=2
 Probability that a score is above 700 is the tail
probability of z=2
 Table 3 provides a probability of 0.4772 between
mean=500 and 700
 z=2
 Right-tail probability for a score of 700 equals
0.5-0.4772=0.0228
 2.28% of the SAT scores are above 700
◦ Now find the probability of having a score below
450
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
The z-score is used to compare values from
different normal distributions
◦ SAT
 μ=500
 σ=100
◦ ACT
 μ=18
 σ=6
x
650  500
zSAT 

 1.5

100
x   25  18
z ACT 

 1.17

6
◦ What is better, 650 on the SAT or 25 on the ACT?
 Corresponding tail probabilities?
 How many percent have worse SAT or ACT scores?
 In other words, 650 and 25 correspond to what
percentiles?
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
The scores on the Psychomotor Development Index
(PDI) are approximately normally distributed with
mean 100 and standard deviation 15. An infant is
selected at random.
◦ Find the probability that the infant’s PDI score is at least
100
 P(X>100)
◦ Find the probability that PDI is between 97 and 103
 P(97<X<103)
◦ Find the z-score for a PDI value of 90
 Would you be surprised to observe a value of 90?
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