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Click the mouse button or press the Space Bar to display the answers. 5-Minute Check on Chapter 8-1b What are the four parts to a confidence interval problem? What three things must the interpretation cover? What three things affect the size of the margin of error? Which two does the analyst have some control over? What is the formula used to solve for sample size required? Parameter, conditions, calculations, interpretation 3 C’s: conclusion, connection (the CI) , context Standard deviation, sample size and confidence level Sample size and confidence level z*σ 2 n ≥ ------- MOE Click the mouse button or press the Space Bar to display the answers.

Estimating a Population Proportion Lesson 8 - 2 Estimating a Population Proportion

Objectives CONSTRUCT and INTERPRET a confidence interval for a population proportion DETERMINE the sample size required to obtain a level C confidence interval for a population proportion with a specified margin of error DESCRIBE how the margin of error of a confidence interval changes with the sample size and the level of confidence C

Vocabulary none new

Proportion Review Important properties of the sampling distribution of a sample proportion p-hat Center: The mean is p. That is, the sample proportion is an unbiased estimator of the population proportion p. Spread: The standard deviation of p-hat is √p(1-p)/n, provided that the population is at least 10 times as large as the sample. Shape: If the sample size is large enough that both np and n(1-p) are at least 10, the distribution of p-hat is approximately Normal.

Sampling Distribution of p-hat Approximately Normal if np ≥10 and n(1-p)≥10

Inference Conditions for a Proportion SRS – the data are from an SRS from the population of interest Independence – individual observations are independent and when sampling without replacement, N > 10n Normality – for a confidence interval, n is large enough so that np and n(1-p) are at least 10 or more

Confidence Interval for P-hat Always in form of PE  MOE where MOE is confidence factor  standard error of the estimate SE = √p(1-p)/n and confidence factor is a z* value

Example 1 The Harvard School of Public Health did a survey of 10.904 US college students and drinking habits. The researchers defined “frequent binge drinking” as having 5 or more drinks in a row three or more times in the past two weeks. According to this definition, 2486 students were classified as frequent binge drinkers. Based on these data, construct a 99% CI for the proportion p of all college students who admit to frequent binge drinking. Parameter: p-hat PE ± MOE p-hat = 2486 / 10904 = 0.228

Example 1 cont Conditions: 1) SRS  2) Normality  3) Independence  shaky np = 2486>10 way more than n(1-p)=8418>10 110,000 students Calculations: p-hat ± z* SE p-hat ± z* √p(1-p)/n 0.228 ± (2.576) √(0.228) (0.772)/ 10904 0.228 ± 0.010 LB = 0.218 < μ < 0.238 = UB Interpretation: We are 99% confident that the true proportion of college undergraduates who engage in frequent binge drinking lies between 21.8 and 23.8 %.

Example 2 We polled n = 500 voters and when asked about a ballot question, 47% of them were in favor. Obtain a 99% confidence interval for the population proportion in favor of this ballot question (α = 0.005) Parameter: p-hat PE ± MOE Conditions: 1) SRS  2) Normality  3) Independence  assumed np = 235>10 way more than n(1-p)=265>10 5,000 voters

Example 2 cont We polled n = 500 voters and when asked about a ballot question, 47% of them were in favor. Obtain a 99% confidence interval for the population proportion in favor of this ballot question (α = 0.005) Calculations: p-hat ± z* SE p-hat ± z* √p(1-p)/n 0.47 ± (2.576) √(0.47) (0.53)/ 500 0.47 ± 0.05748 0.41252 < p < 0.52748 Interpretation: We are 99% confident that the true proportion of voters who favor the ballot question lies between 41.3 and 52.7 %.

Sample Size Needed for Estimating the Population Proportion p The sample size required to obtain a (1 – α) * 100% confidence interval for p with a margin of error E is given by z* n = p(1 - p) ------ E 2 rounded up to the next integer, where p is a prior estimate of p. If a prior estimate of p is unavailable, the sample required is z* n = 0.25 ------ E 2 rounded up to the next integer. The margin of error should always be expressed as a decimal when using either of these formulas

Example 3 In our previous polling example, how many people need to be polled so that we are within 1 percentage point with 99% confidence? Since we do not have a previous estimate, we use p = 0.5 z * n = 0.25 ------ E 2 MOE = E = 0.01 Z* = Z .995 = 2.575 2.575 n = 0.25 -------- = 16,577 0.01 2

Summary and Homework Summary Homework Point Estimate (PE)  Margin of Error (MOE) PE is an unbiased estimator of the population parameter MOE is confidence level  standard error (SE) of the estimator SE is in the form of standard deviation / √sample size Homework Problems 35, 37, 41, 43, 47