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The Normal Probability Distribution

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Distribution Chapter What is the standard deviation? s = (25.5) = 5.0498. What is the probability that less than 40 homes in the sample have video recorders? – PowerPoint PPT presentation

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Title: The Normal Probability Distribution

1
• The Normal Probability
Distribution

Chapter
2
THIS CHAPTERS GOALS
• TO LIST THE CHARACTERISTICS OF THE NORMAL
DISTRIBUTION.
• TO DEFINE AND CALCULATE Z VALUES.
• TO DETERMINE PROBABILITIES ASSOCIATED WITH THE
STANDARD NORMAL DISTRIBUTION.
• TO USE THE NORMAL DISTRIBUTION TO APPROXIMATE THE
BINOMIAL DISTRIBUTION.

3
CHARACTERISTICS OF A NORMAL PROBABILITY
DISTRIBUTION
• The normal curve is bell-shaped and has a single
peak at the exact center of the distribution.
• The arithmetic mean, median, and mode of the
distribution are equal and located at the peak.
• Half the area under the curve is above this
center point, and the other half is below it.
• The normal probability distribution is
• It is asymptotic - the curve gets closer and
closer to the x-axis but never actually touches
it.

4
CHARACTERISTICS OF A NORMAL DISTRIBUTION
Normal curve is symmetrical - two halves
identical -
Tail
Tail
0.5
0.5
Theoretically, curve extends to - infinity
Theoretically, curve extends to infinity
Mean, median, and mode are equal
5
Normal Distributions with Equal Means but
Different Standard Deviations.
s 3.1 s 3.9 s 5.0
m 20
6
Normal Probability Distributions with Different
Means and Standard Deviations.
m 5, s 3 m 9, s 6 m 14, s 10
7
THE STANDARD NORMAL PROBABILITY DISTRIBUTION
• A normal distribution with a mean of 0 and a
standard deviation of 1 is called the standard
normal distribution.
• z value The distance between a selected value,
designated X, and the population mean m, divided
by the population standard deviation, s.
• The z-value is the number of standard deviations
X is from the mean.

8
EXAMPLE
• The monthly incomes of recent MBA graduates in a
large corporation are normally distributed with a
mean of 2,000 and a standard deviation of 200.
What is the z value for an income X of 2,200?
1,700?
• For X 2,200 and since z (X - m)/s, then
z (2,200
- 2,000)/200 1.
• A z value of 1 indicates that the value of 2,200
is 1 standard deviation above the mean of 2,000.

9
EXAMPLE (continued)
• For X 1,700 and since z (X - m)/s, then
z (1,700 - 2,000)/200 -1.5.
• A z value of -1.5 indicates that the value of
2,200 is 1.5 standard deviation below the mean
of 2,000.
• How might a corporation use this type of
information?

10
AREAS UNDER THE NORMAL CURVE
• About 68 percent of the area under the normal
curve is within plus one and minus one standard
deviation of the mean. This can be written as
m 1s.
• About 95 percent of the area under the normal
curve is within plus and minus two standard
deviations of the mean, written m 2s.
• Practically all (99.74 percent) of the area under
the normal curve is within three standard
deviations of the mean, written m 3s.

11
Between 1. 68.26 2. 95.44 3. 99.97
m
m1s
m2s
m3s
m-1s
m-2s
m3s
12
P(z)?
• A typical need is to determine the probability of
a z-value being greater than or less than some
value.
• Tabular Lookup (Appendix D, page 474)
• EXCEL Function NORMSDIST(z)

13
EXAMPLE
• The daily water usage per person in Toledo, Ohio
is normally distributed with a mean of 20 gallons
and a standard deviation of 5 gallons.
• About 68 of the daily water usage per person in
Toledo lies between what two values?

14
EXAMPLE
• The daily water usage per person in Toledo (X),
Ohio is normally distributed with a mean of 20
gallons and a standard deviation of 5 gallons.
• What is the probability that a person selected at
random will use less than 20 gallons per day?
• What is the probability that a person selected at
random will use more than 20 gallons per day?

15
EXAMPLE (continued)
• What percent uses between 20 and 24 gallons?
• The z value associated with X 20 is z 0 and
with X 24, z (24 - 20)/5 0.8
P(20 lt X lt 24) P(0 lt z lt 0.8)
0.288128.81
• What percent uses between 16 and 20 gallons?

16
P(0 lt z lt 0.8) 0.2881
0.8
17
EXAMPLE (continued)
• What is the probability that a person selected at
random uses more than 28 gallons?

18
P(z gt 1.6) 0.5 - 0.4452 0.0048
Area 0.4452
z
1.6
19
EXAMPLE (continued)
• What percent uses between 18 and 26 gallons?

20
Total area 0.1554 0.3849 0.5403
Area 0.1554
Area 0.3849
z
- .4
1.2
21
EXAMPLE (continued)
• How many gallons or more do the top 10 of the
population use?
• Let X be the least amount. Then we need to find
Y such that P(X ³ Y) 0.1 To find the
corresponding z value look in the body of the
table for (0.5 - 0.1) 0.4. The corresponding z
value is 1.28 Thus we have (Y - 20)/5 1.28,
from which Y 26.4. That is, 10 of the
population will be using at least 26.4 gallons
daily.

22
(Y - 20)/5 1.28
Y 26.4
0.4
0.1
z
1.28
23
EXAMPLE
• A professor has determined that the final
averages in his statistics course is normally
distributed with a mean of 72 and a standard
deviation of 5. He decides to assign his grades
for his current course such that the top 15 of
the students receive an A. What is the lowest
average a student must receive to earn an A?

24
(Y - 72)/5 1.04
Y 77.2
0.35
0.15
z
1.04
25
EXAMPLE
• The amount of tip the waiters in an exclusive
restaurant receive per shift is normally
distributed with a mean of 80 and a standard
deviation of 10. A waiter feels he has provided
poor service if his total tip for the shift is
less than 65. Based on his theory, what is the
probability that he has provided poor service?

26
Area 0.5 - 0.4332 0.0668
Area 0.4332
z
- 1.5
27
THE NORMAL APPROXIMATION TO THE BINOMIAL
• Using the normal distribution (a continuous
distribution) as a substitute for a binomial
distribution (a discrete distribution) for large
values of n seems reasonable because as n
increases, a binomial distribution gets closer
and closer to a normal distribution.
• When to use the normal approximation?
• The normal probability distribution is generally
deemed a good approximation to the binomial
probability distribution when np and n(1 - p) are
both greater than 5.

28
Binomial Distribution with n 3 and p 0.5.
P(r)
0.5
0.4
0.3
0.25
r
0
1
2
29
Binomial Distribution with n 5 and p 0.5.
P(r)
r
30
Binomial Distribution with n 20 and p 0.5.
P(r)
Observe the Normal shape.
r
31
THE NORMAL APPROXIMATION (continued)
• Recall for the binomial experiment
• There are only two mutually exclusive outcomes
(success or failure) on each trial.
• A binomial distribution results from counting the
number of successes.
• Each trial is independent.
• The probability p is fixed from trial to trial,
and the number of trials n is also fixed.

32
CONTINUITY CORRECTION FACTOR
• The value 0.5 subtracted or added, depending on
the problem, to a selected value when a binomial
probability distribution, which is a discrete
probability distribution, is being approximated
by a continuous probability distribution--the
normal distribution.
• The basic concept is that a slice of the normal
curve from x-0.5 to x0.5 is approximately equal
to P(x).

33
EXAMPLE
• A recent study by a marketing research firm
showed that 15 of the homes had a video recorder
for recording TV programs. A sample of 200 homes
is obtained. (Let X be the number of homes).
• Of the 200 homes sampled how many would you
expect to have video recorders?
• m np (0.15)(200) 30 n(1 - p) 170
• What is the variance?
• s2 np(1 - p) (30)(1- 0.15) 25.5

34
EXAMPLE (continued)
• What is the standard deviation?
• s Ö(25.5) 5.0498.
• What is the probability that less than 40 homes
in the sample have video recorders?
• We need P(X lt 40) P(X 39). So, using the
normal approximation, P(X 39.5)
• Pz (39.5 - 30)/5.0498 P(z
1.8812)
• P(z 1.88) 0.5 0.4699 0.9699
• Why did I use 39.5 ? ...
• How would you calculate P(X39) ?

35
P(z 1.88) 0.5 0.4699 0.9699
0.5
0.4699
z
1.88
36
EXAMPLE (continued)
• What is the probability that more than 24 homes
in the sample have video recorders?

37
P(z ³ -1.09) 0.5 0.3621 0.8621.
0.5
0.3621
z
-1.09
38
EXAMPLE (continued)
• What is the probability that exactly 40 homes in
the sample have video recorders?

39
P(1.88 z 2.08) 0.4812 - 0.4699 0.0113
1.88
2.08
0.4699
0.4812
z