1. Normal as Approximation to Binomial
Section 6-6
Normal as Approximation to Binomial

2. Learning Targets
This section presents a method for using a normal distribution as an approximation to the binomial probability distribution.
If the conditions of np ≥ 5 and nq ≥ 5 are both satisfied, then probabilities from a binomial probability distribution can be approximated well by using a normal distribution with mean μ = np and standard deviation

3. Remember Binomial Probability Distribution
There must be a fixed number of trials.
Each trial must be independent.
Every trial must have all outcomes classified into two categories (commonly, success and failure).
The probability of each outcome remains consistent throughout every trial.

4. New Notes for Binomials

5. Procedure for Using a Normal Distribution to Approximate a Binomial Distribution

6. Procedure for Using a Normal Distribution to Approximate a Binomial Distribution
4. Draw a normal distribution centered about , then draw a vertical strip area centered over x. Mark the left side of the strip with the number equal to x – 0.5, and mark the right side with the number equal to x Consider the entire area of the entire strip to represent the probability of the discrete whole number itself.

7. Procedure for Using a Normal Distribution to Approximate a Binomial Distribution
5. Determine whether the value of x itself is included in the probability. Determine whether you want the probability of at least x, at most x, more than x, fewer than x, or exactly x. Shade the area to the right or left of the strip; also shade the interior of the strip if and only if x itself is to be included. This total shaded region corresponds to the probability being sought.

8. Procedure for Using a Normal Distribution to Approximate a Binomial Distribution
6. Using x – 0.5 or x in place of x, find the area of the shaded region: find the z score; use that z score to find the area to the left of the adjusted value of x; use that cumulative area to identify the shaded area corresponding to the desired probability.

9. (that is, adding and subtracting 0.5).
Definition
When we use the normal distribution (which is a continuous probability distribution) as an approximation to the binomial distribution (which is discrete), a continuity correction is made to a discrete whole number x in the binomial distribution by representing the discrete whole number x by the interval from
x – 0.5 to x + 0.5
(that is, adding and subtracting 0.5).
page 295 of Elementary Statistics, 10th Edition

10. Continuity Correction
X represents the number of successes which is discrete, however normal distributions are for continuous random variables.
As a result, we must do a “continuity correction”, in which we create a 1 unit interval around x, by considering x and x + 0.5

11. The How To Finding the Probability of
“Less than120 Men” Among 213 Passengers
119.5
= 100
120
For x < 120: Calculate the area to the left of 119.5

12. The How To Finding the Probability of
“At most 120 Men” Among 213 Passengers
120.5
= 100
120
For x < 120: Calculate the area to the left of 120.5

13. The How To Finding the Probability of
“More than120 Men” Among 213 Passengers
120.5
= 100
120
For x > 120: Calculate the area to the right of 120.5

14. The How To Finding the Probability of
“At least 120 Men” Among 213 Passengers
119.5
= 100
120
For x > 120: Calculate the area to the right of 119.5

15. The How To Finding the Probability of
“Has 120 Men” Among 213 Passengers
119.5
120.5
= 100
120
For x = 120: Calculate the area between and 120.5

16. x = at least 8 (X ≥ 8) x = more than 8 (X > 8)
(includes 8 and above)
x = more than 8 (X > 8)
(doesn’t include 8)
x = at most 8 (X ≤ 8)
(includes 8 and below)
x = fewer than 8 (X < 8)
(doesn’t include 8)
x = exactly 8 (X = 8)

17. Overall Process
When using the normal distribution to approximate the binomial distribution, just remember “Paint the integers”

18. Example 1
If a gender-selection technique is tested with 500 couples that have 1 baby, find the probability of getting at least 275 girls.
If the couples did have 275 girls, what would this suggest?

19. Solution

20. Solution (Continued)

21. Example 2

22. Example 2

23. Example 3

24. Example 4

25. Example 5
If you flip a coin 20 times, what is the probability of it landing on heads exactly 10 times?

26. Recap In this section we have discussed:
Approximating a binomial distribution with a normal distribution.
Procedures for using a normal distribution to approximate a binomial distribution.
Continuity corrections.

27. Homework
Pg #5-9,13-14, 21(a, b, d), 27