1. Confidence Intervals about a Population Proportion Section 8.3
Alan Craig

2. Objectives 8.3 Obtain a point estimate for the population proportion
Obtain and interpret a confidence interval for the population proportion
Determine the sample size for estimating a population proportion

3. Point Estimate of a Population Proportion
Suppose a simple random sample of size n is obtained from a population in which each individual either does or does not have a certain characteristic. The best point estimate of p, denoted , the proportion of the population with a certain characteristic, is given by
where x is the number of individuals in the sample with the specified characteristic.

4. Example: #8 (a), p. 374
A study of 74 patients with ulcers was conducted in which they were prescribed 40 mg of Pepcid. After 8 weeks, 58 reported confirmed ulcer healing.
(a) Obtain a point estimate for the proportion of patients with ulcers receiving Pepcid who will have confirmed ulcer healing.

5. Example: #8 (a), p. 374
A study of 74 patients with ulcers was conducted in which they were prescribed 40 mg of Pepcid. After 8 weeks, 58 reported confirmed ulcer healing.
(a) Obtain a point estimate for the proportion of patients with ulcers receiving Pepcid who will have confirmed ulcer healing.

6. Sampling Distribution of
For a simple random sample of size n such that n ≤ .05N (i.e., sample size is no more than 5% of the population), the sampling distribution of is approximately normal with
mean
and standard deviation
provided that np(1-p) ≥ 10.

7. Constructing a (1-a) ·100% Confidence Interval for a Population Proportion
For a simple random sample of size n, a
(1-a) ·100% confidence interval for p is given by
provided that np(1-p) ≥ 10.

8. Example: #8, (b), p.374
(b) Verify that the requirements for constructing a confidence interval about are satisfied.
What do we need to do?

9. Example: #8, (b), p.374
(b) Verify that the requirements for constructing a confidence interval about are satisfied.
We must show that np(1-p) ≥ 10.
74 * * ( ) = > 10

10. Example: #8, (c), p.374
(c) Construct a 99% confidence interval for the proportion of patients with ulcers receiving Pepcid who will have confirmed ulcer healing.

11. Example: #8, (c), p.374
(c) Construct a 99% confidence interval for the proportion of patients with ulcers receiving Pepcid who will have confirmed ulcer healing.

12. Example: #8, (c), p.374
Construct a 99% confidence interval for the proportion of patients with ulcers receiving Pepcid who will have confirmed ulcer healing.
Using Calculator: STATTESTSA: 1-PropZInt
Enter 58 for x, 74 for n, and .99 for C-Level

13. Margin of Error  Sample Size
Solving margin of error to find sample size gives

14. Margin of Error  Sample Size
So we can use a prior estimate for p,
or we can find the largest value of
Using the fact that this is a parabola that opens down (see Figure 17 p. 373), we can find the y-coordinate of the vertex—that is its maximum value
Alternatively, we can use Calculus to find the maximum value.
In either case ≤ 0.25, so

15. Sample Size for Estimating the Population Proportion p
The sample of size needed for a (1-a) ·100% confidence interval for p with a margin of error E is given by
(rounded up to next integer) where is a prior estimate of p. If a prior estimate of p is unavailable, the sample size required is

16. Example: # 16, p. 375
An urban economist wishes to estimate the percentage of Americans who own their house. What size sample should be obtained if he wishes the estimate to be within 2 percentage points with 90% confidence if
(a) he uses a Census Bureau estimate of 67.5% from the 4th quarter of 2000?
(b) he does not use any prior estimates?

17. Example: # 16, p. 375
within 2 percentage points with 90% confidence if
(a) he uses a Census Bureau estimate of 67.5% from the 4th quarter of 2000?

18. Example: # 16, p. 375
within 2 percentage points with 90% confidence if
(b) he does not use any prior estimates?

19. Questions
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