Continuous Random Variables and The Normal Distribution: Prem Mann, Introductory Statistics, 7/E

CHAPTER 6
CONTINUOUS RANDOM
VARIABLES AND THE
NORMAL
DISTRIBUTION
Prem Mann, Introductory Statistics, 7/E

Copyright © 2010 John Wiley & Sons. All right reserved
CONTINUOUS PROBABILITY
DISTRIBUTION

Figure 6.1 Histogram and polygon for Table 6.1.

Figure 6.2 Probability distribution curve for
heights.

CONTINUOUS PROBABILITY
DISTRIBUTION
Two characteristics
1. The probability that x assumes a value

in any interval lies in the range 0 to 1
2. The total probability of all the (mutually
exclusive) intervals within which x can
assume a value of 1.0

Figure 6.3 Area under a curve between two
points.

Figure 6.4 Total area under a probability
distribution curve.

Figure 6.5 Area under the curve as probability.

Figure 6.6 Probability that x lies in the interval
65 to 68 inches.

Figure 6.7 The probability of a single value of x
is zero.

Figure 6.8 Probability “from 65 to 68” and
“between 65 and 68”.

THE NORMAL DISTRIBUTION
 Normal Probability Distribution
 A normal probability distribution ,

when plotted, gives a bell-shaped curve
such that:
1. The total area under the curve is 1.0.
2. The curve is symmetric about the mean.
3. The two tails of the curve extend indefinitely.

Figure 6.11 Normal distribution with mean μ and
standard deviation σ.

Figure 6.12 Total area under a normal curve.

Figure 6.13 A normal curve is symmetric about
the mean.

Figure 6.14 Areas of the normal curve beyond
μ ± 3σ.

Figure 6.15 Three normal distribution curves
with the same mean but different standard
deviations.
x
Figure 6.16 Three normal distribution curves
with different means but the same standard
deviation.

THE STANDARD NORMAL DISTRIBTUION
 Definition
 The normal distribution with μ = 0 and σ = 1
is called the standard normal
distribution.

Figure 6.17 The standard normal distribution
curve.

THE STANDARD NORMAL DISTRIBTUION
z Values or z Scores
 Definition
 Theunits marked on the horizontal axis of

the standard normal curve are denoted by z
and are called the z values or z scores. A
specific value of z gives the distance between
the mean and the point represented by z in
terms of the standard deviation.
Figure 6.18 Area under the standard normal
curve.

Example 6-1
 Findthe area under the standard normal

curve between when z = 1.95.

Table 6.2 Area Under the Standard Normal
Curve to the Left of z = 1.95

Figure 6.19 Area to the left of z = 1.95.

Example 6-2
 Find
the area under the standard normal
curve from z = -2.17 to z = 0.

Example 6-2: Solution
 To find the area from z=-2.17 to z =0 , first

we find the areas to the left of z=0 and to
the left of z=-2.17 in Table IV. As shown in
Table 6.3, these two areas are .5 and .
0150, respectively. Next we subtract .0150
from .5 to find the required area.
 Area from -2.17 to 0 = P(-2.17≤ z ≤ 0)
= .5000 - .0150 = .4850

Table 6.3 Area Under the Standard Normal
Curve

Figure 6.20 Area from z = -2.17 to z = 0.

Example 6-3
 Find the following areas under the

standard normal curve.
a) Area to the right of z = 2.32
b) Area to the left of z = -1.54

a) To find the area to the right of z=2.32,

first we find the area to the left of
z=2.32. Then we subtract this area from
1.0, which is the total area under the
curve. The required area is 1.0 - .9898 =
.0102.

Figure 6.21 Area to the right of z = 2.32.

b) To find the area under the standard

normal curve to the left of z=-1.54, we
find the area in Table IV that
corresponds to -1.5 in the z column
and .04 in the top row. This area is .
0618. Area to the left of -1.54
= P (z < -1.54) =.0618

Figure 6.22 Area to the left of z = -1.54.

Example 6-4
 Find the following probabilities for the

a) P (1.19 < z < 2.12)
b) P (-1.56 < z < 2.31)
c) P (z > -.75)

a) P (1.19 < z < 2.12)

= Area between 1.19 and 2.12
= .9830 - .8830
= .1000

Figure 6.23 Finding P (1.19 < z < 2.12).

b) P (-1.56 < z < 2.31)
= Area between -1.56 and 2.31
= .9896 - .0594
= .9302

Figure 6.24 Finding P (-1.56 < z < 2.31).

c) P (z > -.75)
= Area to the right of -.75
= 1.0 - .2266
= .7734

Figure 6.25 Finding P (z > -.75).

Figure 6.26 Area within one standard deviation
of the mean.

Figure 6.27 Area within two standard deviations
of the mean.

Figure 6.28 Area within three standard
deviations of the mean.

Example 6-5
Find the following probabilities for the

a) P (0 < z < 5.67)
b) P (z < -5.35)

a) P (0 < z < 5.67)

= Area between 0 and 5.67
= 1.0 - .5
= .5 approximately

Figure 6.29 Area between z = 0 and z = 5.67.

b) P (z < -5.35)
= Area to the left of -5.35
= .00 approximately

Figure 6.30 Area to the left of z = -5.35.

STANDARDIZING A NORMAL
DISTRIBUTION
 Converting an x Value to a z Value

 For a normal random variable x, a particular value
of x can be converted to its corresponding z value
by using the formula
x
z

 where μ and σ are the mean and standard
deviation of the normal distribution of x,
respectively.
Example 6-6
 Let x be a continuous random variable that

has a normal distribution with a mean of
50 and a standard deviation of 10. Convert
the following x values to z values and find
the probability to the left of these points.
a) x = 55
b) x = 35

a) x = 55
x 55  50
z   .50
 10
P(x < 55) = P(z < .50) = .6915

Figure 6.31 z value for x = 55.

b) x = 35
x   35  50
z   1.50
 10
P(x < 35) = P(z < -1.50) = .0668

Figure 6.32 z value for x = 35.

Example 6-7
Let x be a continuous random variable

that is normally distributed with a mean
of 25 and a standard deviation of 4.
Find the area
a) between x = 25 and x = 32
b) between x = 18 and x = 34

a)
 The z value for x = 25 is 0
 The z value for x = 32 is
x   32  25
z   1.75
 4
 P (25 < x < 32) = P(0 < z < 1.75)
= .4599

Figure 6.33 Area between x = 25 and x = 32.

b)
 For x = 18: z  18  25  1.75
4
For x = 34: z 
34  25
  2.25
4
 P (18 < x < 34) = P (-1.75 < z < 2.25 )

= .9878 - .0401 = .9477

Example 6-8
Let x be a normal random variable with

its mean equal to 40 and standard
deviation equal to 5. Find the following
probabilities for this normal distribution
a) P (x > 55)
b) P (x < 49)

a)
 For x = 55:
55  40
z  3.00
5
 P (x > 55) = P (z > 3.00) = 1.0 - .9987
= .0013

Figure 6.35 Finding P (x > 55).

b)
 For x = 49:
49  40
z  1.80
5
 P (x < 49) = P (z < 1.80) = .9461

Figure 6.36 Finding P (x < 49).

Example 6-9
Let x be a continuous random variable that

has a normal distribution with μ = 50 and σ
= 8. Find the probability P (30 ≤ x ≤ 39).

 For x = 30: 30  50
z  2.50
8
 For x = 39: 39  50
z  1.38
8
 P (30 ≤ x ≤ 39) = P (-2.50 ≤ z ≤ -1.38)
= .0838 - .0062 = .0776
Figure 6.37 Finding P (30 ≤ x ≤ 39).

Example 6-10
Let x be a continuous random variable

that has a normal distribution with a
mean of 80 and a standard deviation of
12. Find the area under the normal
distribution curve
a) from x = 70 to x = 135
b) to the left of 27

a)
For x = 70: 70  80
 z  .83
12
 For x = 135: z  135  80  4.58
12
 P (70 ≤ x ≤ 135) = P (-.83 ≤ z ≤ 4.58)
= 1 - .2033
= .7967
approximately

b)
 For x = 27: z  27  80  4.42
12
 P (x < 27) = P (z < -4.42)
=.00 approximately

Figure 6.39 Area to the left of x = 27.

APPLICATIONS OF THE NORMAL DISTRIBUTION
Section 6.2 through 6.4 discussed the

normal distribution, how to convert a
normal distribution to the standard normal
distribution, and how to find areas under a
normal distribution curve. This section
presents examples that illustrate the
applications of the normal distribution.

Example 6-11
According to a Sallie Mae and credit bureau
data, in 2008, college students carried an
average of $3173 debt on their credit cards
(USA TODAY, April 13, 2009). Suppose
that current credit card debts for all college
students have a normal distribution with a
mean of $3173 and a standard deviation of
$800. Find the probability that credit card
debt for a randomly selected college
student is between $2109 and $3605.
2109  3173
 For x = $2109: z  1.33
800
 For x = $3605: 3605  3173
z  .54
800
 P ($2109 < x < $3605)
= P (-1.33 < z < .54)
= .7054 - .0918
= .6136 = 61.36%

Figure 6.40 Area between x = $2109 and x =
$3605.

Example 6-12
A racing car is one of the many toys

manufactured by Mack Corporation. The
assembly times for this toy follow a normal
distribution with a mean of 55 minutes and
a standard deviation of 4 minutes. The
company closes at 5 p.m. every day. If
one worker starts to assemble a racing car
at 4 p.m., what is the probability that she
will finish this job before the company
closes for the day?
 For x = 60:
60  55
z  1.25
4
 P(x ≤ 60) = P(z ≤ 1.25) = .8944
 Thus, the probability is .8944 that this

worker will finish assembling this racing
car before the company closes for the day.

Example 6-13
 Hupper Corporation produces many types of soft

drinks, including Orange Cola. The filling machines
are adjusted to pour 12 ounces of soda into each
12-ounce can of Orange Cola. However, the actual
amount of soda poured into each can is not exactly
12 ounces; it varies from can to can. It has been
observed that the net amount of soda in such a
can has a normal distribution with a mean of 12
ounces and a standard deviation of .015 ounce.

Example 6-13
a) What is the probability that a randomly

selected can of Orange Cola contains 11.97 to
11.99 ounces of soda?
b) What percentage of the Orange Cola cans
contain 12.02 to 12.07 ounces of soda?

a)
 For x = 11.97: 11 .97  12
z  2.00
.015
 For x = 11.99: 11 .99  12
z  .67
.015
 P (11.97 ≤ x ≤ 11.99)
= P (-2.00 ≤ z ≤ -.67) = .2514 - .0228
= .2286

Figure 6.42 Area between x = 11.97 and x =
11.99.

b)
 For x = 12.02: 12.02  12
z  1.33
.015
 For x = 12.07: 12.07  12
z  4.67
.015
 P (12.02 ≤ x ≤ 12.07)
= P (1.33 ≤ z ≤ 4.67) = 1 - .9082
= .0918

Figure 6.43 Area from x = 12.02 to x = 12.07.

Example 6-14
The life span of a calculator manufactured by

Texas Instruments has a normal distribution with a
mean of 54 months and a standard deviation of 8
months. The company guarantees that any
calculator that starts malfunctioning within 36
months of the purchase will be replaced by a new
one. About what percentage of calculators made
by this company are expected to be replaced?

 For x = 36: 36  54
z  2.25
8
 P(x < 36) = P (z < -2.25)
= .0122
 Hence, 1.22% of the calculators are

expected to be replaced.

DETERMINING THE z AND x VALUES WHEN AN AREA
UNDER THE NORMAL DISTRIBUTION CURVE IS KNOWN
Now we learn how to find the corresponding

value of z or x when an area under a normal
distribution curve is known.

Example 6-15
Find a point z such that the area under the

standard normal curve to the left of z is .
9251.

Figure 6.45 Finding the z value.

Table 6.4 Finding the z Value When Area Is
Known.

Example 6-16
Find the value of z such that the area

under the standard normal curve in the
right tail is .0050.

 Area to the left of z = 1.0 - .0050 = .9950

 Look for .9950 in the body of the normal
distribution table. Table VII does not
contain .9950.
 Find the value closest to .9950, which is
either .9949 or .9951.
 If we choose .9951, the z = 2.58.
 If we choose .0049, the z = 2.57.


Example 6-17
Find the value of z such that the area under

the standard normal curve in the left tail
is .05.

 Because .05 is less than .5 and it is the

area in the left tail, the value of z is
negative.
 Look for .0500 in the body of the
normal distribution table. The value
closes to .0500 in Table IV is either .
0505 or .0495.
 If we choose .0495, the z = -1.65.


Finding an x Value for a Normal Distribution
For a normal curve, with known values of μ

and σ and for a given area under the curve
to the left of x, the x value is calculated as
x = μ + zσ

Example 6-18
Recall Example 6-14. It is known that the
life of a calculator manufactured by Texas
Instruments has a normal distribution with
a mean of 54 months and a standard
deviation of 8 months. What should the
warranty period be to replace a
malfunctioning calculator if the company
does not want to replace more than 1% of
all the calculators sold?

 Area to the left of x = .01 or 1%
 Find the z value from the normal
distribution table for .0100. Table IV does
not contain a value that is exactly .0100.
 The value closest to .0100 in the table is .
0099. The z = -2.33.
 x = μ + zσ = 54 + (-2.33)(8)
= 54 – 18.64 = 35.36

Thus, the company should replace all

calculators that start to malfunction within
35.36 months (which can be rounded to 35
months) of the date of purchase so that
they will not have to replace more than 1%
of the calculators.

Figure 6.48 Finding an x value.

Example 6-19
Almost all high school students who intend to go to
college take the SAT test. In a recent test, the
mean SAT score (in verbal and mathematics) of all
students was 1020. Debbie is planning to take this
test soon. Suppose the SAT scores of all students
who take this test with Debbie will have a normal
distribution with a mean of 1020 and a standard
deviation of 153. What should her score be on this
test so that only 10% of all examinees score
higher than she does?

 Area to the left of the x value
= 1.0 - .10 = .9000
 Look for .9000 in the body of the normal
distribution table. The value closest to .9000 in
Table IV is .8997, and the z value is 1.28.
 x = μ + zσ = 1020 + 1.28(153)
= 1020 + 195.84 = 1215.84 ≈ 1216
 Thus, if Debbie scores 1216 on the SAT, only
about 10% of the examinees are expected to
score higher than she does.

Figure 6.49 Finding an x value.

THE NORMAL APPROXIMATION OF THE
BINOMIAL DISTRIBUTION
1. The binomial distribution is applied to a discrete

random variable.
2. Each repetition, called a trial, of a binomial
experiment results in one of two possible
outcomes, either a success or a failure.
3. The probabilities of the two (possible) outcomes
remain the same for each repetition of the
experiment.
4. The trials are independent.

 The binomial formula, which gives the

probability of x successes in n trials, is
n x
P( x)  n C x p q x

Normal Distribution as an Approximation to
Binomial Distribution
Usually, the normal distribution is used as

an approximation to the binomial
distribution when np and nq are both greater
than 5 -- that is, when
np > 5 and nq > 5

Table 6.5 The Binomial Probability Distribution
for n = 12 and p = .50

Figure 6.50 Histogram for the probability
distribution of Table 6.5.

Example 6-20
According to an estimate, 50% of the

people in the United States have at least
one credit card. If a random sample of 30
persons is selected, what is the probability
that 19 of them will have at least one
credit card?

 n = 30, p = .50, q = 1 – p = .50
 x = 19, n – x = 30 – 19 = 11
 From the binomial formula,
P (19)  30 C19 (.5) (.5)  .0509

19 11

 Let’s solve this problem using the normal
distribution as an approximation to the
binomial distribution.
 np = 30(.50) = 15 > 5 and
nq = 30(.50) = 15 > 5.
 Can use the normal distribution as an
approximation to solve this binomial
problem.

 Step 1. Compute μ and σ for the
binomial distribution.
  np  30(.50)  15
  npq  30(.50)(.50)  2.73861279
 Step 2. Convert the discrete random
variable into a continuous random
variable (by making the correction for
continuity).
Continuity Correction Factor
Continuity Correction Factor
Definition
The addition of .5 and/or subtraction of .5
from the value(s) of x when the normal
distribution is used as an approximation to
the binomial distribution, where x is the
number of successes in n trials, is called
the continuity correction factor.
Figure 6.51

 Step 3. Compute the required probability
using the normal distribution.
18.5  15
 For x = 18.5: z  1.28
2.73861279
19.5  15
 For x = 19.5: z  1.64
2.73861279
 P(18.5 ≤ x ≤ 19.5) = P(1.28 ≤ z ≤ 1.64)
= .9495 - .8997 = .0498

 Thus, based on the normal approximation,

the probability that 19 persons in a sample
of 30 will have at lease on credit card is
approximately .0498.
 Using the binomial formula, we obtain the
exact probability .0509.
 The error due to using the normal
approximation is .0509 - .0498 = .0011.

Figure 6.52 Area between x = 18.5 and x = 19.5.

Example 6-21
According to a joint reader survey by
USATODAY.com and TripAdvisor.com, 34% of the
people surveyed said that the first thing they do
after checking into a hotel is to adjust the
thermostat (USA TODAY, June 12, 2009).
Suppose that this result is true for the current
population of all adult Americans who stay in
hotels. What is the probability that in a random
sample of 400 adult Americans who stay in
hotels, 115 to 130 will say that the first thing
they do after checking into a hotel is to adjust the
thermostat?

 n = 400, p = .34, q = 1 – .34 = .66
  np  400(.34)  136
  npq  400(.34)(.66)  9.47417543
 For x = 114.5: 114.5  136
z  2.27
9.47417543
 For x = 130.5 130.5  136

z  .58
9.47417543

 P(114.5 ≤ x ≤ 130.5)
= P(-2.27 ≤ z ≤ -.58)
= .2810 - .0116 = .2694
 Thus, the probability that 115 to 130

adults in a sample of 400 will say that
the first thing they do after checking into
a hotel is to adjust the thermostat is

Figure 6.53 Area between x = 114.5 and x =
130.5

Example 6-22
According to an American Laser Centers survey,

32% of adult men said that their stomach is the
least favorite part of their body (USA TODAY,
March 10, 2009). Assume that this percentage is
true for the current population of all men. What is
the probability that 170 or more adult in a random
sample of 500 will say that their stomach is the
least favorite part of their body?

 n = 500, p = .32, q = 1 – .32 = .68
  np  500(.32)  160
  npq  500(.32)(.68)  10.43072385
 For x = 169.5:
169.5  160
z  .91
10.430772385

 P(x ≥ 169.5) = P(z ≥ .91)
= 1.0 - .8186 = .1814
 Thus, the probability that 170 170 or

more adult in a random sample of 500
will say that their stomach is the least
favorite part of their body is

Figure 6.54 Area to the right of x = 169.5


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CHAPTER 6

Prem Mann, Introductory Statistics, 7/E

Prem Mann, Introductory Statistics, 7/E

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1. The probability that x assumes a value

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 Normal Probability Distribution

 A normal probability distribution ,

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 Theunits marked on the horizontal axis of

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 Findthe area under the standard normal

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 To find the area from z=-2.17 to z =0 , first

= .5000 - .0150 = .4850

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 Find the following areas under the

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a) To find the area to the right of z=2.32,

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b) To find the area under the standard

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 Find the following probabilities for the

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a) P (1.19 < z < 2.12)

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Find the following probabilities for the

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a) P (0 < z < 5.67)

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