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Lecture6 -2 - Hypothesis Test 2 - Hypothesis Tests II...

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Unformatted text preview: Hypothesis Tests II AS&W – Chapter 9 Population Proportion Hypothesis Testing and Decision Making Calculating the Probability of Type II Errors Determining the Sample Size for Hypothesis Tests About a Population Mean 1 A Summary of Forms for Null and Alternative Hypotheses About a Population Proportion The equality part of the hypotheses always appears in the null hypothesis. In general, a hypothesis test about the value of a population proportion p must take one of the following three forms (where p0 is the hypothesized value of the population proportion). H 0: p p0 H a: p p0 H 0: p p0 H a: p p0 H 0: p p0 H a: p p0 One-tailed One-tailed (lower tail) (upper tail) Two-tailed 2 Tests About a Population Proportion Test Statistic p p0 z p where: p0 (1 p0 ) p n assuming np > 5 and n(1 – p) > 5 3 Tests About a Population Proportion Rejection Rule: p –Value Approach Reject H0 if p –value < Rejection Rule: Critical Value Approach H0: pp0 Reject H0 if z > z H0: pp Reject H0 if z < -z H0: pp Reject H0 if z < -z2 or z > z 4 Two-Tailed Test About a Population Proportion Example: National Safety Council For New Year’s week, the National Safety Council estimated that 500 people would be killed and 25,000 injured on the nation’s roads. The NSC claimed that 50% of the accidents would be caused by drunk driving. 5 Two-Tailed Test About a Population Proportion Example: National Safety Council A sample of 120 accidents showed that 67 were caused by drunk driving. Use these data to test the NSC’s claim with = .05. 6 Two-Tailed Test About a Population Proportion p –Value and Critical Value Approaches H 0: p .5 1. Determine the hypotheses. H a: p .5 = .05 2. Specify the level of significance. 3. Compute the value of the test statistic. a common error is pusing in this formula p0(1 p0 ) .5(1 .5) p .045644 n 120 p p0 (67/ 120) .5 z 1.28 p .045644 7 Two-Tailed Test About a Population Proportion pValue Approach 4. Compute the p -value. For z = 1.28, cumulative probability = .8997 p–value = 2(1 .8997) = .2006 5. Determine whether to reject H0. Because p–value = .2006 > = .05, we cannot reje 8 Two-Tailed Test About a Population Proportion Critical Value Approach 4. Determine the criticals value and rejection rule. For /2 = .05/2 = .025, z.025 = 1.96 Reject H0 if z < -1.96 or z > 1.96 5. Determine whether to reject H0. Because 1.278 > -1.96 and < 1.96, we cannot rejec 9 Hypothesis Testing and Decision Making In many decision-making situations the decision maker may want, and in some cases may be forced, to take action with both the conclusion do not reject H0 and the conclusion reject H0. In such situations, it is recommended that the hypothesis-testing procedure be extended to include consideration of making a Type II error. 10 Calculating the Probability of a Type II Error in Hypothesis Tests About a Population 1. Formulate the null Mean and alternative hypotheses. 2. Using the critical value approach, use the level of significance to determine the critical value and the rejection rule for the test. 3. Using the rejection rule, solve for the value of the sample mean corresponding to the critical value of the test statistic. 11 Calculating the Probability of a Type II Error in Hypothesis Tests About a Population 4. Use the results fromMean step 3 to state the values of the sample mean that lead to the acceptance of H0; this defines the acceptance region. 5. Using the sampling distributionx of for a value of satisfying the alternative hypothesis, and the accep region from step 4, compute the probability that the sample mean will be in the acceptance region. (Thi the probability of making a Type II error at the chose level of .) 12 Calculating the Probability of a Type II Error Example: Metro EMS (revisited) Recall that the response times for a random sample of 40 medical emergencies were tabulated. The sample mean is 13.25 minutes. The population standard deviation is believed to be 3.2 minutes. The EMS director wants to perform a hypothesis test, with a .05 level of significance, to determine whether or not the service goal of 12 minutes or less is being achieved. 13 Calculating the Probability of a Type II Error 1. Hypotheses are: H0: 12 and H rule is: Reject H0 if z > 1.645 2.a: Rejection 3. Value of the sample mean that identifies the rejection region: x z 3.2/ 12 1.645 40 3.2 x 12 1.645 12.8323 40 4. We will accept H0 when x < 12.8323 14 Calculating the Probability of a Type II Error 5. Probabilities that the sample mean will be in the acceptance region: 12.8323 z Values of 3.2/ 40 14.0 13.6 13.2 12.8323 12.8 12.4 12.0001 -2.31 -1.52 -0.73 0.00 0.06 0.85 1.645 .0104 .0643 .2327 .5000 .5239 .8023 .9500 .9896 .9357 .7673 .5000 .4761 .1977 .0500 15 Calculating the Probability of a Type II Error Calculating the Probability of a Type II Error Observations about the preceding table: When the true population mean is close to the null hypothesis value of 12, there is a high probability that we will make a Type II error. Example: = 12.0001, = .9500 When the true population mean is far above the null hypothesis value of 12, there is a low Example: = 14.0, = .0104 probability that we will make a Type II error. 16 Power of the Test The probability of correctly rejecting H0 when it is false is called the power of the test. For any particular value of , the power is 1 – . We can show graphically the power associated with each value of ; such a graph is called a power curve. (See next slide.) 17 Power Curve Probability of Correctly Rejecting Null Hypothesis 1.00 0.90 0.80 H0 False 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00 11.5 12.0 12.5 13.0 13.5 14.0 14.5 18 Determining the Sample Size for a Hypothesis Test About a Population Mean The specified level of significance determines the probability of making a Type I error. By controlling the sample size, the probability of making a Type II error is controlled. 19 Determining the Sample Size for a Hypothesis Test About a Population Mean Sampling distribution of x when H0 is true and = 0 c Reject H0 x 0 Note: x H0: 0 Ha: n Sampling distribution of x when H0 is false and a > 0 c a x 20 Determining the Sample Size for a Hypothesis Test About a Population Mean n ( z z ) 2 2 ( 0 a )2 where z = z value providing an area of in the tail z = z value providing an area of in the tail = population standard deviation 0 = value of the population mean in H0 a = value of the population mean used for the Type II error Note: In a two-tailed hypothesis test, use z /2 not z 21 Relationship Among , , and n Once two of the three values are known, the other can be computed. For a given level of significance , increasing the sample size n will reduce . For a given sample size n, decreasing will increase , whereas increasing will decrease . 22 Determining the Sample Size for a Hypothesis Test About a Population Mean Let’s assume that the director of medical services makes the following statements about the allowable probabilities for the Type I and Type II errors: •If the mean response time is = 12 minutes, I am willing to risk an = .05 probability of rejecting H0. response time is 0.75 minutes over • If the mean the specification ( = 12.75), I am willing to risk a = .10 probability of not rejecting H0. 23 Determining the Sample Size for a Hypothesis Test About a Population Mean = .05, = .10 z = 1.645, z = 1.28 0 = 12, a = 12.75 = 3.2 ( z z )2 2 (1.645 1.28) 2 (3.2) 2 n 155.75 156 2 2 ( 0 a ) (12 12.75) 24 ...
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