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Copyright © 2011 Pearson Education, Inc. Statistical Tests Chapter 16

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16.1 Concepts of Statistical Tests A manager is evaluating software to filter SPAM e-mails (cost $15,000). To make it profitable, the software must reduce SPAM to less than 20%. Should the manager buy the software? Use a statistical test to answer this question Consider the plausibility of a specific claim (claims are called hypotheses) Copyright © 2011 Pearson Education, Inc. 3 of 40

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16.1 Concepts of Statistical Tests Null and Alternative Hypotheses Statistical hypothesis: claim about a parameter of a population. Null hypothesis (H 0 ): specifies a default course of action, preserves the status quo. Alternative hypothesis (H a ): contradicts the assertion of the null hypothesis. Copyright © 2011 Pearson Education, Inc. 4 of 40

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16.1 Concepts of Statistical Tests SPAM Software Example Let p = email that slips past the filter H 0 : p ≥ 0.20 H a : p < 0.20 These hypotheses lead to a one-sided test. Copyright © 2011 Pearson Education, Inc. 5 of 40

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16.1 Concepts of Statistical Tests One- and Two-Sided Tests One-sided test: the null hypothesis allows any value of a parameter larger (or smaller) than a specified value. Two-sided test: the null hypothesis asserts a specific value for the population parameter. Copyright © 2011 Pearson Education, Inc. 6 of 40

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16.1 Concepts of Statistical Tests Type I and II Errors Reject H 0 incorrectly (buying software that will not be cost effective) Retain H 0 incorrectly (not buying software that would have been cost effective) Copyright © 2011 Pearson Education, Inc. 7 of 40

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16.1 Concepts of Statistical Tests Type I and II Errors indicates a correct decision Copyright © 2011 Pearson Education, Inc. 8 of 40

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16.1 Concepts of Statistical Tests Other Tests Visual inspection for association, normal quantile plots and control charts all use tests of hypotheses. For example, the null hypothesis in a visual test for association is that there is no association between two variables shown in the scatterplot. Copyright © 2011 Pearson Education, Inc. 9 of 40

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16.1 Concepts of Statistical Tests Sampling Distribution Statistical tests rely on the sampling distribution of the statistic that estimates the parameter specified in the null and alternative hypotheses. Key question: What is the chance of getting a sample that differs from H 0 by as much as this one if H 0 is true? Copyright © 2011 Pearson Education, Inc. 10 of 40

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16.2 Testing the Proportion SPAM Software Example Based on n = 100, = 0.11. Assuming H 0 is true, the sampling distribution of is approximately normal with mean p = 0.20 and SE( ) = 0.04 (note that the hypothesized value p 0 = 0.20 is used to calculate SE). Copyright © 2011 Pearson Education, Inc. 11 of 40

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16.2 Testing the Proportion SPAM Software Example What is the chance of making a Type I error? Possible sampling distributions for. Chance of a Type I error shown in shaded area. Copyright © 2011 Pearson Education, Inc. 12 of 40

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16.2 Testing the Proportion z–Test and p-Value p-Value: the largest chance of a Type I error if H 0 is rejected based on the observed test statistic. z-Test: test of H 0 based on a count of the standard errors separating H 0 from the test statistic. Copyright © 2011 Pearson Education, Inc. 13 of 40

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16.2 Testing the Proportion z–Test for SPAM Software Example = -2.25 Copyright © 2011 Pearson Education, Inc. 14 of 40

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16.2 Testing the Proportion p–Value for SPAM Software Example Interpret the p-value as a weight of evidence against H 0 ; small values mean that H 0 is not plausible. Copyright © 2011 Pearson Education, Inc. 15 of 40

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16.2 Testing the Proportion α-Value α-Value: threshold that sets the maximum tolerance for a Type I error. Statistically significant: data contradict the null hypothesis and lead us to reject H 0 (p-value < α). The p-value in the SPAM example is less than the typical α of 0.05; should buy the software. Copyright © 2011 Pearson Education, Inc. 16 of 40

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16.2 Testing the Proportion Type II Error Power: probability that a test rejects H 0. If a test has little power when H 0 is false, it is likely to miss meaningful deviations from the null hypothesis and produce a Type II error. Copyright © 2011 Pearson Education, Inc. 17 of 40

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16.2 Testing the Proportion Summary Copyright © 2011 Pearson Education, Inc. 18 of 40

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16.2 Testing the Proportion Checklist SRS condition: the sample is a simple random sample from the relevant population. Sample size condition (for proportion): both np 0 and n(1 - p 0 ) are larger than 10. Copyright © 2011 Pearson Education, Inc. 19 of 40

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4M Example 16.1: DO ENOUGH HOUSEHOLDS WATCH? Motivation The Burger King ad featuring Coq Roq won critical acclaim. In a sample of 2,500 homes, MediaCheck found that only 6% saw the ad. An ad must be viewed by 5% or more of households to be effective. Based on these sample results, should the local sponsor run this ad? Copyright © 2011 Pearson Education, Inc. 20 of 40

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4M Example 16.1: DO ENOUGH HOUSEHOLDS WATCH? Mehod Set up the null and alternative hypotheses. H 0 : p ≤ 0.05 H a : p > 0.05 Use α = 0.05. Note that p is the population proportion who watch this ad. Both SRS and sample size conditions are met. Copyright © 2011 Pearson Education, Inc. 21 of 40

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4M Example 16.1: DO ENOUGH HOUSEHOLDS WATCH? Mechanics Perform a one-sided z-test for a proportion. z = 2.3 with p-value of 0.011 Reject H 0. Copyright © 2011 Pearson Education, Inc. 22 of 40

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4M Example 16.1: DO ENOUGH HOUSEHOLDS WATCH? Message The results are statistically significant. We can conclude that more than 5% of households watch this ad. The Burger King Coq Roq ad is cost effective and should be run. Copyright © 2011 Pearson Education, Inc. 23 of 40

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16.3 Testing the Mean Similar to Tests of Proportions The hypothesis test of µ replaces with. Unlike the test of proportions, σ is not specified. Use s from the sample as an estimate of σ to calculate the estimated standard error of. Copyright © 2011 Pearson Education, Inc. 24 of 40

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16.3 Testing the Mean Example: Denver Rental Properties A firm is considering expanding into the Denver area. In order to cover costs, the firm needs rents in this area to average more than $500 per month. Are Denver rents high enough to justify the expansion? Copyright © 2011 Pearson Education, Inc. 25 of 40

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16.3 Testing the Mean Null and Alternative Hypotheses Let µ = mean monthly rent for all rental properties in the Denver area Set up hypotheses as: H 0 : µ ≤ µ 0 = $500 H a : µ > µ 0 = $500 Copyright © 2011 Pearson Education, Inc. 26 of 40

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16.3 Testing the Mean t - Statistic Used in the t-test for µ (since s estimates σ) The t-statistic, with n-1 df, is Copyright © 2011 Pearson Education, Inc. 27 of 40

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16.3 Testing the Mean Example: Denver Rental Properties The firm obtained rents for a sample of size n=45; the average rent was $647 with s = $299. t = 3.298 with 44 df; p-value = 0.00097 Reject H 0 ; mean rent exceeds break-even value. Copyright © 2011 Pearson Education, Inc. 28 of 40

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16.3 Testing the Mean Finding the p-Value in the t-Table t = 3.298 is larger than any value in the row Copyright © 2011 Pearson Education, Inc. 29 of 40

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16.3 Testing the Mean Summary Copyright © 2011 Pearson Education, Inc. 30 of 40

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16.3 Testing the Mean Checklist SRS condition: the sample is a simple random sample from the relevant population. Sample size condition. Unless the population is normally distributed, a normal model can be used to approximate the sampling distribution of if n is larger than 10 times both the squared skewness and absolute value of kurtosis. Copyright © 2011 Pearson Education, Inc. 31 of 40

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4M Example 16.2: COMPARING RETURNS ON INVESTMENTS Motivation Does stock in IBM return more, on average, than T-Bills? From 1980 through 2005, T- Bills returned 5% each month. Copyright © 2011 Pearson Education, Inc. 32 of 40

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4M Example 16.2: COMPARING RETURNS ON INVESTMENTS Method Let µ = mean of all future monthly returns for IBM stock. Set up the hypotheses as H 0 : µ ≤ 0.005 H a : µ > 0.005 Sample consists of monthly returns on IBM for 312 months (January 1980 – December 2005) Copyright © 2011 Pearson Education, Inc. 33 of 40

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4M Example 16.2: COMPARING RETURNS ON INVESTMENTS Mechanics Sample yields = 0.0106 with s = 0.0805. t = 1.22 with 311 df; p-value = 0.111 Copyright © 2011 Pearson Education, Inc. 34 of 40

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4M Example 16.2: COMPARING RETURNS ON INVESTMENTS Message Monthly IBM returns from 1980 through 2005 do not bring statistically significantly higher earnings than comparable investments in US Treasury Bills during this period. Copyright © 2011 Pearson Education, Inc. 35 of 40

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16.4 Other Properties of Tests Significance versus Importance Statistical significance does not mean that you have made an important or meaningful discovery. The size of the sample affects the p-value of a test. With enough data, a trivial difference from H 0 leads to a statistically significant outcome. Copyright © 2011 Pearson Education, Inc. 36 of 40

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16.4 Other Properties of Tests Confidence Interval or Test? A confidence interval provides a range of parameter values that are compatible with the observed data. A test provides a precise analysis of a specific hypothesized value for a parameter. Copyright © 2011 Pearson Education, Inc. 37 of 40

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Best Practices Pick the hypotheses before looking at the data. Choose the null hypothesis on the basis of profitability. Pick the α level first, taking into account both types of error. Think about whether α = 0.05 is appropriate for each test. Copyright © 2011 Pearson Education, Inc. 38 of 40

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Best Practices (Continued) Make sure to have an SRS from the right population. Use a one-sided test. Report a p–value to summarize the outcome of a test. Copyright © 2011 Pearson Education, Inc. 39 of 40

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Pitfalls Do not confuse statistical significance with substantive importance. Do not think that the p–value is the probability that the null hypothesis is true. Avoid cluttering a test summary with jargon. Copyright © 2011 Pearson Education, Inc. 40 of 40

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