1. Normal Probability Distributions
Chapter 5
Normal Probability Distributions
Larson/Farber 4th ed

2. Chapter Outline
5.1 Introduction to Normal Distributions and the Standard Normal Distribution
5.2 Normal Distributions: Finding Probabilities
5.3 Normal Distributions: Finding Values
5.4 Sampling Distributions and the Central Limit Theorem
5.5 Normal Approximations to Binomial Distributions
Larson/Farber 4th ed

3. Normal Approximations to Binomial Distributions
Section 5.5
Normal Approximations to Binomial Distributions
Larson/Farber 4th ed

4. Section 5.5 Objectives
Determine when the normal distribution can approximate the binomial distribution
Find the correction for continuity
Use the normal distribution to approximate binomial probabilities
Larson/Farber 4th ed

5. Normal Approximation to a Binomial
The normal distribution is used to approximate the binomial distribution when it would be impractical to use the binomial distribution to find a probability.
Normal Approximation to a Binomial Distribution
If np  5 and nq  5, then the binomial random variable x is approximately normally distributed with
mean μ = np
standard deviation
Larson/Farber 4th ed

6. Normal Approximation to a Binomial
Binomial distribution: p = 0.25
As n increases the histogram approaches a normal curve.
Larson/Farber 4th ed

7. Example: Approximating the Binomial
Decide whether you can use the normal distribution to approximate x, the number of people who reply yes. If you can, find the mean and standard deviation.
Fifty-one percent of adults in the U.S. whose New Year’s resolution was to exercise more achieved their resolution. You randomly select 65 adults in the U.S. whose resolution was to exercise more and ask each if he or she achieved that resolution.
Larson/Farber 4th ed

8. Solution: Approximating the Binomial
You can use the normal approximation
n = 65, p = 0.51, q = 0.49
np = (65)(0.51) = ≥ 5
nq = (65)(0.49) = ≥ 5
Mean: μ = np = 33.15
Standard Deviation:
Larson/Farber 4th ed

9. Example: Approximating the Binomial
Decide whether you can use the normal distribution to approximate x, the number of people who reply yes. If you can find, find the mean and standard deviation.
Fifteen percent of adults in the U.S. do not make New Year’s resolutions. You randomly select 15 adults in the U.S. and ask each if he or she made a New Year’s resolution.
Larson/Farber 4th ed

10. Solution: Approximating the Binomial
You cannot use the normal approximation
n = 15, p = 0.15, q = 0.85
np = (15)(0.15) = 2.25 < 5
nq = (15)(0.85) = ≥ 5
Because np < 5, you cannot use the normal distribution to approximate the distribution of x.
Larson/Farber 4th ed

11. Correction for Continuity
The binomial distribution is discrete and can be represented by a probability histogram.
To calculate exact binomial probabilities, the binomial formula is used for each value of x and the results are added.
Geometrically this corresponds to adding the areas of bars in the probability histogram.
Larson/Farber 4th ed

12. Correction for Continuity
When you use a continuous normal distribution to approximate a binomial probability, you need to move 0.5 unit to the left and right of the midpoint to include all possible x-values in the interval (correction for continuity).
Exact binomial probability
P(x = c) c
Normal approximation
P(c – 0.5 < x < c + 0.5)
c+ 0.5
c– 0.5
Larson/Farber 4th ed

13. Example: Using a Correction for Continuity
Use a correction for continuity to convert the binomial intervals to a normal distribution interval.
The probability of getting between 270 and 310 successes, inclusive.
Solution:
The discrete midpoint values are 270, 271, …, 310.
The corresponding interval for the continuous normal distribution is
269.5 < x < 310.5
Larson/Farber 4th ed

14. Example: Using a Correction for Continuity
Use a correction for continuity to convert the binomial intervals to a normal distribution interval.
The probability of getting at least 158 successes.
Solution:
The discrete midpoint values are 158, 159, 160, ….
The corresponding interval for the continuous normal distribution is
x > 157.5
Larson/Farber 4th ed

15. Example: Using a Correction for Continuity
Use a correction for continuity to convert the binomial intervals to a normal distribution interval.
The probability of getting less than 63 successes.
Solution:
The discrete midpoint values are …,60, 61, 62.
The corresponding interval for the continuous normal distribution is
x < 62.5
Larson/Farber 4th ed

16. Using the Normal Distribution to Approximate Binomial Probabilities
In Words In Symbols
Verify that the binomial distribution applies.
Determine if you can use the normal distribution to approximate x, the binomial variable.
Find the mean  and standard deviation for the distribution.
Specify n, p, and q.
Is np  5? Is nq  5?
Larson/Farber 4th ed

17. Using the Normal Distribution to Approximate Binomial Probabilities
In Words In Symbols
Apply the appropriate continuity correction. Shade the corresponding area under the normal curve.
Find the corresponding z-score(s).
Find the probability.
Add or subtract 0.5 from endpoints.
Use the Standard Normal Table.
Larson/Farber 4th ed

18. Example: Approximating a Binomial Probability
Fifty-one percent of adults in the U. S. whose New Year’s resolution was to exercise more achieved their resolution. You randomly select 65 adults in the U. S. whose resolution was to exercise more and ask each if he or she achieved that resolution. What is the probability that fewer than forty of them respond yes? (Source: Opinion Research Corporation)
Solution:
Can use the normal approximation (see slide 89)
μ = 65∙0.51 = 33.15
Larson/Farber 4th ed

19. Solution: Approximating a Binomial Probability
Apply the continuity correction:
Fewer than 40 (…37, 38, 39) corresponds to the continuous normal distribution interval x < 39.5
39.5
μ =33.15
P(x < 39.5)
Normal Distribution
μ = σ = 4.03 x
1.58
μ =0
P(z < 1.58)
Standard Normal
μ = 0 σ = 1 z
0.9429
P(z < 1.58) =
Larson/Farber 4th ed

20. Example: Approximating a Binomial Probability
A survey reports that 86% of Internet users use Windows® Internet Explorer ® as their browser. You randomly select 200 Internet users and ask each whether he or she uses Internet Explorer as his or her browser. What is the probability that exactly 176 will say yes? (Source: 0neStat.com)
Solution:
Can use the normal approximation
np = (200)(0.86) = 172 ≥ 5 nq = (200)(0.14) = 28 ≥ 5
μ = 200∙0.86 = 172
Larson/Farber 4th ed

21. Solution: Approximating a Binomial Probability
Apply the continuity correction:
Exactly 176 corresponds to the continuous normal distribution interval < x < 176.5
176.5
μ =172
P(175.5 < x < 176.5)
Normal Distribution
μ = σ = 4.91 x
175.5
0.92
μ =0
P(0.71 < z < 0.92)
Standard Normal
μ = 0 σ = 1 z
0.71
0.8212
0.7611
P(0.71 < z < 0.92) = – =
Larson/Farber 4th ed

22. Section 5.5 Summary
Determined when the normal distribution can approximate the binomial distribution
Found the correction for continuity
Used the normal distribution to approximate binomial probabilities
Larson/Farber 4th ed