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Confidence Intervals about a Population Proportion Section 8.3 Alan Craig 770-274-5242 acraig@gpc.edu

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2 Objectives 8.3 1.Obtain a point estimate for the population proportion 2.Obtain and interpret a confidence interval for the population proportion 3.Determine the sample size for estimating a population proportion

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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.

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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.

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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.

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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.

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

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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?

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9 (b) Verify that the requirements for constructing a confidence interval about are satisfied. We must show that np (1- p ) ≥ 10. 74 * 0.784 * (1 - 0.784) = 12.53 > 10 Example: #8, (b), p.374

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

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

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12 (c)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 Example: #8, (c), p.374

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

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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

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15 The sample of size needed for a (1- ) ·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 Sample Size for Estimating the Population Proportion p

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16 (a)he uses a Census Bureau estimate of 67.5% from the 4 th quarter of 2000? (b)he does not use any prior estimates? 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

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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 4 th quarter of 2000?

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

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19 Questions ???????????????

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