# 8 6 X 2 Test for a Variance or Standard Deviation

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1 Section 8 6 x 2 Test for a Variance or Standard Deviation 437 This test uses the P-value method. Therefore, it is not necessary to enter a significance level. 1. Select MegaStat>Hypothesis Tests>Proportion vs. Hypothesized Value. 2. Enter the observed sample proportion 0.37 and the hypothesized proportion Enter the sample size 100 for n. 4. Select not equal for the Alternative and click [OK]. 8 6 X 2 Test for a Variance or Standard Deviation Objective 8 Test variances or standard deviations, using the chi-square test. In Chapter 7, the chi-square distribution was used to construct a confidence interval for a single variance or standard deviation. This distribution is also used to test a claim about a single variance or standard deviation. To find the area under the chi-square distribution, use Table G in Appendix C. There are three cases to consider: 1. Finding the chi-square critical value for a specific a when the hypothesis test is right-tailed. 2. Finding the chi-square critical value for a specific a when the hypothesis test is left-tailed. 3. Finding the chi-square critical values for a specific a when the hypothesis test is two-tailed. Example 8 21 Find the critical chi-square value for 15 degrees of freedom when a 0.05 and the test is right-tailed. The distribution is shown in Figure Figure 8 30 Chi-Square Distribution for Example Find the a value at the top of Table G, and find the corresponding degrees of freedom in the left column. The critical value is located where the two columns meet in this case, See Figure

2 438 Chapter 8 Hypothesis Testing Figure 8 31 Locating the Critical Value in Table G for Example 8 21 Degrees of freedom Example 8 22 Find the critical chi-square value for 10 degrees of freedom when a 0.05 and the test is left-tailed. This distribution is shown in Figure Figure 8 32 Chi-Square Distribution for Example When the test is left-tailed, the a value must be subtracted from 1, that is, The left side of the table is used, because the chi-square table gives the area to the right of the critical value, and the chi-square statistic cannot be negative. The table is set up so that it gives the values for the area to the right of the critical value. In this case, 95% of the area will be to the right of the value. For 0.95 and 10 degrees of freedom, the critical value is See Figure Figure 8 33 Locating the Critical Value in Table G for Example 8 22 Degrees of freedom Example 8 23 Find the critical chi-square values for 22 degrees of freedom when a 0.05 and a twotailed test is conducted. 8 48

3 Section 8 6 x 2 Test for a Variance or Standard Deviation 439 When a two-tailed test is conducted, the area must be split, as shown in Figure Note that the area to the right of the larger value is (0.05 2), and the area to the right of the smaller value is ( ). Figure 8 34 Chi-Square Distribution for Example Remember that chi-square values cannot be negative. Hence, one must use a values in the table of and With 22 degrees of freedom, the critical values are and , respectively. After the degrees of freedom reach 30, Table G gives values only for multiples of 10 (40, 50, 60, etc.). When the exact degrees of freedom one is seeking are not specified in the table, the closest smaller value should be used. For example, if the given degrees of freedom are 36, use the table value for 30 degrees of freedom. This guideline keeps the type I error equal to or below the a value. When one is testing a claim about a single variance using the chi-square test, there are three possible test situations: right-tailed test, left-tailed test, and two-tailed test. If a researcher believes the variance of a population to be greater than some specific value, say, 225, then the researcher states the hypotheses as H 0 : s and H 1 : s and conducts a right-tailed test. If the researcher believes the variance of a population to be less than 225, then the researcher states the hypotheses as H 0 : s and H 1 : s and conducts a left-tailed test. Finally, if a researcher does not wish to specify a direction, he or she states the hypotheses as H 0 : s and H 1 : s and conducts a two-tailed test. Formula for the Chi-Square Test for a Single Variance x 2 n 1 s 2 s 2 with degrees of freedom equal to n 1 and where n sample size s 2 sample variance s 2 population variance 8 49

4 440 Chapter 8 Hypothesis Testing One might ask, Why is it important to test variances? There are several reasons. First, in any situation where consistency is required, such as in manufacturing, one would like to have the smallest variation possible in the products. For example, when bolts are manufactured, the variation in diameters due to the process must be kept to a minimum, or the nuts will not fit them properly. In education, consistency is required on a test. That is, if the same students take the same test several times, they should get approximately the same grades, and the variance of each of the student s grades should be small. On the other hand, if the test is to be used to judge learning, the overall standard deviation of all the grades should be large so that one can differentiate those who have learned the subject from those who have not learned it. Three assumptions are made for the chi-square test, as outlined here. U nusual Stat About 20% of cats owned in the United States are overweight. Assumptions for the Chi-Square Test for a Single Variance 1. The sample must be randomly selected from the population. 2. The population must be normally distributed for the variable under study. 3. The observations must be independent of one another. The traditional method for hypothesis testing follows the same five steps listed earlier. They are repeated here. Step 1 Step 2 Step 3 Step 4 Step 5 State the hypotheses and identify the claim. Find the critical value(s). Compute the test value. Make the decision. Summarize the results. Examples 8 24 through 8 26 illustrate the traditional hypothesis-testing procedure for variances. Example 8 24 An instructor wishes to see whether the variation in scores of the 23 students in her class is less than the variance of the population. The variance of the class is 198. Is there enough evidence to support the claim that the variation of the students is less than the population variance (s 2 225) at a 0.05? Assume that the scores are normally distributed. Step 1 State the hypotheses and identify the claim. H 0 : s and H 1 : s (claim) Step 2 Find the critical value. Since this test is left-tailed and a 0.05, use the value The degrees of freedom are n Hence, the critical value is Note that the critical region is on the left, as shown in Figure

5 Section 8 6 x 2 Test for a Variance or Standard Deviation 441 Figure 8 35 Critical Value for Example Step 3 Compute the test value. x 2 n 1 s2 s Step 4 Make the decision. Since the test value falls in the noncritical region, as shown in Figure 8 36, the decision is to not reject the null hypothesis. Figure 8 36 Critical and Test Values for Example Step 5 Summarize the results. There is not enough evidence to support the claim that the variation in test scores of the instructor s students is less than the variation in scores of the population. Example 8 25 A hospital administrator believes that the standard deviation of the number of people using outpatient surgery per day is greater than 8. A random sample of 15 days is selected. The data are shown. At a 0.10, is there enough evidence to support the administrator s claim? Assume the variable is normally distributed Step 1 State the hypotheses and identify the claim. H 0 : s 2 64 and H 1 : s 2 64 (claim) Since the standard deviation is given, it should be squared to get the variance. Step 2 Find the critical value. Since this test is right-tailed with d.f. of and a 0.10, the critical value is Step 3 Compute the test value. Since raw data are given, the standard deviation of the sample must be found by using the formula in Chapter 3 or your calculator. It is s x 2 n 1 s2 s

6 442 Chapter 8 Hypothesis Testing Step 4 Make the decision. The decision is to reject the null hypothesis since the test value, 27.44, is greater than the critical value, , and falls in the critical region. See Figure Figure 8 37 Critical and Test Value for Example Step 5 Summarize the results. There is enough evidence to support the claim that the standard deviation is greater than 8. Example 8 26 A cigarette manufacturer wishes to test the claim that the variance of the nicotine content of its cigarettes is Nicotine content is measured in milligrams, and assume that it is normally distributed. A sample of 20 cigarettes has a standard deviation of 1.00 milligram. At a 0.05, is there enough evidence to reject the manufacturer s claim? Step 1 State the hypotheses and identify the claim. Step 2 H 0 : s (claim) and H 1 : s Find the critical values. Since this test is a two-tailed test at a 0.05, the critical values for and must be found. The degrees of freedom are 19; hence, the critical values are and 8.907, respectively. The critical or rejection regions are shown in Figure Figure 8 38 Critical Values for Example Step 3 Step 4 Compute the test value. n x 2 1 s s Since the standard deviation s is given in the problem, it must be squared for the formula. Make the decision. Do not reject the null hypothesis, since the test value falls between the critical values ( ) and in the noncritical region, as shown in Figure

7 Section 8 6 x 2 Test for a Variance or Standard Deviation 443 Figure 8 39 Critical and Test Values for Example Step 5 Summarize the results. There is not enough evidence to reject the manufacturer s claim that the variance of the nicotine content of the cigarettes is equal to Approximate P-values for the chi-square test can be found by using Table G in Appendix C. The procedure is somewhat more complicated than the previous procedures for finding P-values for the z and t tests since the chi-square distribution is not exactly symmetric and x 2 values cannot be negative. As we did for the t test, we will determine an interval for the P-value based on the table. Examples 8 27 through 8 29 show the procedure. Example 8 27 Find the P-value when x , n 8, and the test is right-tailed. To get the P-value, look across the row with d.f. 7 in Table G and find the two values that falls between. They are and Look up to the top row and find the a values corresponding to and They are 0.01 and 0.005, respectively. See Figure Hence the P-value is contained in the interval P-value (The P-value obtained from a calculator is ) Figure 8 40 P-Value Interval for Example 8 27 Degrees of freedom * falls between and

8 444 Chapter 8 Hypothesis Testing Example 8 28 Find the P-value when x , n 13, and the test is left-tailed. To get the P-value, look across the row with d.f. 12 and find the two values that falls between. They are and Look up to the top row and find the values corresponding to and They are 0.99 and 0.975, respectively. When the x 2 test value falls on the left side, each of the values must be subtracted from 1 to get the interval that P-value falls between and Hence the P-value falls in the interval 0.01 P-value (The P-value obtained from a calculator is ) When the x 2 test is two-tailed, both interval values must be doubled. If a two-tailed test were being used in Example 8 28, then the interval would be 2(0.01) P-value 2(0.025), or 0.02 P-value The P-value method for hypothesis testing for a variance or standard deviation follows the same steps shown in the preceding sections. Step 1 Step 2 Step 3 Step 4 Step 5 State the hypotheses and identify the claim. Compute the test value. Find the P-value. Make the decision. Summarize the results. Example 8 29 shows the P-value method for variances or standard deviations. Example 8 29 A researcher knows from past studies that the standard deviation of the time it take to inspect a car is 16.8 minutes. A sample of 24 cars is selected and inspected. The standard deviation was 12.5 minutes. At a 0.05, can it be concluded that the standard deviation has changed? Use the P-value method. Step 1 Step 2 Step 3 State the hypotheses and identify the claim. H 0 : s 16.8 and H 1 : s 16.8 (claim) Compute the test value. x 2 n 1 s s Find the P-value. Using Table G with d.f. 23, the value falls between and , corresponding to and 0.95, respectively. Since these values are found on the left side of the distribution, each value must be subtracted from 1. Hence and Since this is a two-tailed test, the area must be doubled to obtain the P-value interval. Hence 0.05 P-value 0.10, or somewhere between 0.05 and (The P-value obtained from a calculator is ) 8 54

9 Section 8 6 x 2 Test for a Variance or Standard Deviation 445 Step 4 Make the decision. Since a 0.05 and the P-value is between 0.05 and 0.10, the decision is to not reject the null hypothesis since P-value a. Step 5 Summarize the results. There is not enough evidence to support the claim that the standard deviation has changed. Applying the Concepts 8 6 Testing Gas Mileage Claims Assume that you are working for the Consumer Protection Agency and have recently been getting complaints about the highway gas mileage of the new Dodge Caravans. Chrysler Corporation agrees to allow you to randomly select 40 of its new Dodge Caravans to test the highway mileage. Chrysler claims that the Caravans get 28 mpg on the highway. Your results show a mean of 26.7 and a standard deviation of 4.2. You support Chrysler s claim. 1. Show why you support Chrysler s claim by listing the P-value from your output. After more complaints, you decide to test the variability of the miles per gallon on the highway. From further questioning of Chrysler s quality control engineers, you find they are claiming a standard deviation of Test the claim about the standard deviation. 3. Write a short summary of your results and any necessary action that Chrysler must take to remedy customer complaints. 4. State your position about the necessity to perform tests of variability along with tests of the means. See page 461 for the answers. Exercises Using Table G, find the critical value(s) for each, show the critical and noncritical regions, and state the appropriate null and alternative hypotheses. Use s a. a 0.05, n 18, right-tailed b. a 0.10, n 23, left-tailed c. a 0.05, n 15, two-tailed d. a 0.10, n 8, two-tailed e. a 0.01, n 17, right-tailed f. a 0.025, n 20, left-tailed g. a 0.01, n 13, two-tailed h. a 0.025, n 29, left-tailed 2. (ans) Using Table G, find the P-value interval for each x 2 test value. a. x , n 16, right-tailed b. x , n 25, left-tailed c. x , n 11, two-tailed d. x , n 9, right-tailed e. x , n 28, two-tailed f. x , n 19, left-tailed g. x , n 6, two-tailed h. x , n 23, two-tailed For Exercises 3 through 9, assume that the variables are normally or approximately normally distributed. Use the traditional method of hypothesis testing unless otherwise specified. 3. A nutritionist claims that the standard deviation of the number of calories in 1 tablespoon of the major brands of pancake syrup is 60. A sample of major brands of syrup is selected, and the number of calories is shown. At a 0.10, can the claim be rejected? Source: Based on information from The Complete Book of Food Counts by Corrine T. Netzer, Dell Publishers, New York. 4. Aresearcher claims that the variance of the number of yearly forest fires in the United States is greater than 140. For a 13-year period, the variance of the number of forest fires in the United States is 146. Test the claim at a What factor would influence the variation of the number of forest fires that occurred in the United States? Source: National Interagency Fire Center. 5. Test the claim that the standard deviation of the number of aircraft stolen each year in the United States is less 8 55

10 446 Chapter 8 Hypothesis Testing than 15 if a sample of 12 years had a standard deviation of Use a Source: Aviation Crime Prevention Institute. 6. A random sample of weights (in pounds) of football players at a local college is listed. At a 0.05, is there sufficient evidence that the standard deviation of all football players at the college is less than 10 pounds? Source: Football Preview, Washington Observer-Reporter. 7. The manager of a large company claims that the standard deviation of the time (in minutes) that it takes a telephone call to be transferred to the correct office in her company is 1.2 minutes or less. A sample of 15 calls is selected, and the calls are timed. The standard deviation of the sample is 1.8 minutes. At a 0.01, test the claim that the standard deviation is less than or equal to 1.2 minutes. Use the P-value method. 8. A machine fills 12-ounce bottles with soda. For the machine to function properly, the standard deviation of the sample must be less than or equal to 0.03 ounce. A sample of eight bottles is selected, and the number of ounces of soda in each bottle is given. At a 0.05, can we reject the claim that the machine is functioning properly? Use the P-value method A random sample of 20 different kinds of doughnuts had the following calorie counts. At a 0.01, is there sufficient evidence to conclude that the standard deviation is greater than 20 calories? Source: The Doctor s Pocket Calorie, Fat, and Carbohydrate Counter. 10. The high temperatures for a random sample of 7 days in July in southwestern Pennsylvania are listed At a 0.10, is there sufficient evidence to conclude that the standard deviation for high temperatures is less than 10? Source: AccuWeather, Washington Observer-Reporter. 11. A researcher claims that the standard deviation of the number of deaths annually from tornadoes in the United States is less than 35. If a sample of 11 randomly selected years had a standard deviation of 32, is the claim believable? Use a Source: National Oceanic and Atmospheric Administration. 12. It has been reported that the standard deviation of the speeds of drivers on Interstate 75 near Findlay, Ohio, is 8 miles per hour for all vehicles. A driver feels from experience that this is very low. A survey is conducted, and for 50 drivers the standard deviation is 10.5 miles per hour. At a 0.05, is the driver correct? 13. A random sample of home run totals for National League Home Run Champions from 1938 to 2001 is shown. At the 0.05 level of significance, is there sufficient evidence to conclude that the variance is greater than 25? Source: N.Y. Times Almanac. 14. Are some fast-food restaurants healthier than others? The number of grams of fat in a regular serving of French fries for a random sample of restaurants is shown. Is there sufficient evidence to conclude that the standard deviation exceeds 4 grams of fat? Use a Source: The Doctor s Pocket Calorie, Fat, and Carbohydrate Counter. Technology Step by Step MINITAB Step by Step Hypothesis Test for Variance For Example 8 25, test the administrator s claim that the standard deviation is greater than 8. There is no menu item to calculate the test statistic and P-value directly. Calculate the Standard Deviation and Sample Size 1. Enter the data into a column of MINITAB. Name the column OutPatients. 2. The standard deviation and sample size will be calculated and stored. a) Select Calc>Column Statistics. b) Check the button for Standard deviation. You can only do one of these statistics at a time. 8 56

11 Section 8 6 x 2 Test for a Variance or Standard Deviation 447 c) Use OutPatients for the Input variable. d) Store the result in s, then click [OK]. 3. Select Edit>Edit Last Dialog Box, then do three things: a) Change the Statistic option from Standard Deviation to N nonmissing. b) Type n in the text box for Store the result. c) Click [OK]. Calculate the Chi-Square Test Statistic 4. Select Calc>Calculator. a) In the text box for Store result in variable: type in K3. The chi-square value will be stored in a constant so it can be used later. b) In the expression, type in the formula as shown. The double asterisk is the symbol used for a power. c) Click [OK]. The chi-square value of will be stored in K3. Calculate the P-Value d) Select Calc>Probability Distributions>Chi-Square. e) Click the button for Cumulative probability. f) Type in 14 for Degrees of freedom. g) Click in the text box for Input constant and type K3. h) Type in K4 for Optional storage. i) Click [OK]. Now K4 contains the area to the left of the chi-square test statistic. Subtract the cumulative area from 1 to find the area on the right side of the chi-square test statistic. This is the P-value for a right-tailed test. j) Select Calc>Calculator. k) In the text box for Store result in variable, type in P-Value. l) The expression 1 K4 calculates the complement of the cumulative area. m) Click [OK]. 8 57

12 448 Chapter 8 Hypothesis Testing The result will be shown in the first row of C2, Since the P-value is less than a, reject the null hypothesis. The standard deviation in the sample is 11.2, the point estimate for the true standard deviation s. TI-83 Plus or TI-84 Plus Step by Step The TI-83 Plus and TI-84 Plus do not have a built-in hypothesis test for the variance or standard deviation. However, the downloadable program named SDHYP is available on your CD and Online Learning Center. Follow the instructions with your CD for downloading the program. Performing a Hypothesis Test for the Variance and Standard Deviation (Data) 1. Enter the values into L Press PRGM, move the cursor to the program named SDHYP, and press ENTER twice. 3. Press 1 for Data. 4. Type L 1 for the list and press ENTER. 5. Type the number corresponding to the type of alternative hypothesis. 6. Type the value of the hypothesized variance and press ENTER. 7. Press ENTER to clear the screen. Example TI8 4 This pertains to Example 8 25 in the text. Test the claim that s 8 for these data Since P-value , we reject H 0 and conclude H 1. Therefore, there is enough evidence to support the claim that the standard deviation of the number of people using outpatient surgery is greater than 8. Performing a Hypothesis Test for the Variance and Standard Deviation (Statistics) 1. Press PRGM, move the cursor to the program named SDHYP, and press ENTER twice. 2. Press 2 for Stats. 3. Type the sample standard deviation and press ENTER. 4. Type the sample size and press ENTER. 5. Type the number corresponding to the type of alternative hypothesis. 6. Type the value of the hypothesized variance and press ENTER. 7. Press ENTER to clear the screen. Example TI8 5 This pertains to Example 8 26 in the text. Test the claim that s , given n 20 and s

13 Section 8 7 Additional Topics Regarding Hypothesis Testing 449 Since P-value , we do not reject H 0 and do not conclude H 1. Therefore, there is not enough evidence to reject the manufacturer s claim that the variance of the nicotine content of the cigarettes is equal to Excel Step by Step Hypothesis Test for the Variance Excel does not have a procedure to conduct a hypothesis test for the variance. However, you may conduct the test of the variance by using the MegaStat Add-in available on your CD and Online Learning Center. If you have not installed this add-in, do so by following the instructions on page 24. This test uses the P-value method. Therefore, it is not necessary to enter a significance level. We will use the summary information from Example Enter a label for the variable: Nicotine in cell A1. 2. Enter the observed variance: 1 in cell A2. 3. Enter the sample size: 20 in cell A3. 4. Select MegaStat>Hypothesis Tests>Chi-Square Variance Test. 5. Select summary input. 6. Type in for the Hypothesized variance and select not equal for Alternative. 7. Click [OK]. 8 7 Additional Topics Regarding Hypothesis Testing In hypothesis testing, there are several other concepts that might be of interest to students in elementary statistics. These topics include the relationship between hypothesis testing and confidence intervals, and some additional information about the type II error. Objective 9 Test hypotheses, using confidence intervals. Confidence Intervals and Hypothesis Testing There is a relationship between confidence intervals and hypothesis testing. When the null hypothesis is rejected in a hypothesis-testing situation, the confidence interval for the mean using the same level of significance will not contain the hypothesized mean. Likewise, when the null hypothesis is not rejected, the confidence interval computed using the same level of significance will contain the hypothesized mean. Examples 8 30 and 8 31 show this concept for two-tailed tests. Example 8 30 Sugar is packed in 5-pound bags. An inspector suspects the bags may not contain 5 pounds. A sample of 50 bags produces a mean of 4.6 pounds and a standard deviation of 0.7 pound. Is there enough evidence to conclude that the bags do not contain 5 pounds as stated at a 0.05? Also, find the 95% confidence interval of the true mean. Now H 0 : m 5 and H 1 : m 5 (claim). The critical values are 1.96 and The test value is z X m s n Since , the null hypothesis is rejected. There is enough evidence to support the claim that the bags do not weigh 5 pounds. 8 59

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