# Estimating and Finding Confidence Intervals

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1 . Activity 7 Estimating and Finding Confidence Intervals Topic 33 (40) Estimating A Normal Population Mean μ (σ Known) A random sample of size 10 from a population of heights that has a normal distribution (with σ = 2.5 inches) is given below (with the sample mean). Store this data in L1. {66.71, 66.27, 62.81, 66.92, 62.91, 71.42, 67.39, 63.79, 65.81, 62.81} = L1 ü = n = 10 What is the 80 percent confidence interval for the population mean? This activity begins with estimating the mean of a population with a normal distribution using a simple random sample from that population. It is important that you read Topic 33 before the other topics in Activity 7 because it explains the functions under the STAT TESTS menu, the notation, and the meaning of a confidence interval. You will be using these things in making other estimates. The last topic in this activity, Topic 39, covers the idea of biased and unbiased estimators with a simulation that explains why we divide by (n-1) when calculating the sample standard deviation. The topic number of the related hypothesis test topic is given in parenthesis after the estimation topic number. For example, the (40) after Topic 33 below indicates that Topic 40 is the related hypothesis test topic for Topic TEXAS INSTRUMENTS INCORPORATED STATISTICS HANDBOOK FOR THE TI-83 73

2 Activity 7, Estimations and Confidence Intervals (cont.) There are two possible methods of input, depending on how the data is presented to you in a problem. Sometimes all the data is given and it can be stored in a list. Sometimes only the summary stats are given, such as ü and n, and you must work with these. This activity shows you how to work with both. 1. Press <TESTS> 7:ZInterval for one of the two possible screens (see screen 1 or 3), depending on what input (Inpt:) is highlighted. 2. Enter the correct values from above, highlight Calculate in the last row, and then press Í for output screen 2 or 4. We are 80 percent confident that the population mean lies between and 66.7 inches. Home Screen Calculations What formula was used by the above procedure? 1. With a confidence level of C = 0.80, there is (1-0.80)/2 = 0.10, or 10 percent of the area in each tail of the probability distribution of sample means. 2. To find the critical z value that divides the upper 10 percent from the lower 90 percent, use y [DISTR] 3:invNorm( 0 Ë 90 Í for 1.28 as shown in screen The margin of error E is 1.28 times the standard deviation of the sample means, or E = 1.28 σ/ (n) = / (10) = 1.01 as shown in screen 6. Note that the margin of error can be calculated from the confidence interval above by subtracting the mean from the larger value of the interval, or = The confidence interval becomes ü ± E, or ± 1.01, or to , or to as before. (1) (2) (3) (4) Note: The slight differences shown in the intervals in screen 2 and 4 are because the sample mean calculated from the Data List had three decimal places (ü = ) while the stats version was rounded to ü = (5) (6) 74 STATISTICS HANDBOOK FOR THE TI TEXAS INSTRUMENTS INCORPORATED

3 The Meaning of Confidence Interval (Simulation) The above sample was randomly generated from a normal distribution with a mean of 65 inches and a standard deviation of 2.5 inches (the first interval in screen 7), so the confidence interval of to did, in fact, contain the population mean. However, we take a sample to estimate the population mean because we do not know it. How confident can we be of our answer? In the above case, we can be 80 percent confident because if we took all possible samples of size 10 and calculated the confidence intervals as above, 80 percent of these intervals calculated would contain the correct population mean. In screens 7-17 we generate just ten samples, and seven of the ten contain the true mean. (We did not expect exactly eight of ten for so few tries.) The intervals that miss the true mean (screens 11, 13, and 17) miss by 1.01, 0.37 and 0.57 inches. The sample means are shown as gaps in the middle of the confidence interval lines drawn beneath the normal plot in screen 18 for the ten samples generated. Only when the sample means are in the shaded regions (10 percent in each tail) will the margin of error not be long enough to reach the population mean (the vertical line at 65 inches). Thus, 80 percent of the sample means in this distribution of sample means will be close enough to the population mean for its confidence interval to contain it. To generate the random samples and calculate the confidence intervals as was done in screens 7-17, proceed as follows. 1. Set the random seed as explained in Topic 21 and shown in the first two lines of screen Complete screen 7 by pressing <PRB> 6:randNorm( 65 2 Ë 5 10) L1 ƒ [:] (above the Ë; ties generating the sample and calculating the confidence together in one statement). 3. Press y [CATALOG]. Notice the A (in the upper right corner of the screen) indicating you are in alpha mode. 4. Press Z to point out ZInterval, and then press Í. This pastes it to the home screen. Finish the line by typing 2 Ë 5 L1 0 Ë Press Í to generate the first sample and calculate the confidence interval (64.671, ) as shown in screen Press Í to take the next sample and calculate the confidence interval (63.237, ) as in screen 9. Continue pressing Í for the rest of the 10 intervals as in screens (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) 1997 TEXAS INSTRUMENTS INCORPORATED STATISTICS HANDBOOK FOR THE TI-83 75

4 Activity 7, Estimations and Confidence Intervals (cont.) Topic 34 (41) Estimating a Population Mean μ (σ Unknown) A random sample of size 10 from a population of heights that has a normal distribution is given below (with the sample mean and the standard deviation). Store this data in L1. {66.71, 66.27, 62.81, 66.92, 62.91, 71.42, 67.39, 63.79, 65.81, 62.81} = L1 ü = 65.68, Sx = 2.72, n = 10 What is the 90 percent confidence interval for the population mean? 1. Press <TESTS> 8:TInterval for one of the two possible screens (see screen 19 or 21), depending on what input (Inpt:) is highlighted. 2. Enter the correct values as above, highlight Calculate in the last row, and then press Í for output screen 20 or 22. We are 90 percent confident that the population mean lies between 64.1 and inches. Home Screen Calculations To find the critical t value with the TI-83, there is no invt like the invnorm, so we will use the equation solver at the end of this topic to show a value of (or you can look it up in a table). The margin of error is calculated very much like in Topic 33, but now Sx is used instead of s for a value of Or, from the previous interval, subtract the mean from the upper interval value ( = 1.58). From screen 23, we also see that the confidence interval is to as above, with the width of the interval = 3.16, or twice the margin of error (2 1.58). (19) (20) (21) (22) (23) 76 STATISTICS HANDBOOK FOR THE TI TEXAS INSTRUMENTS INCORPORATED

5 Notice the interval above is wider than the one shown in screen 24 for an 80 percent confidence interval ( = 2.37). (The Inpt screen is not shown, but is like screen 19 with C-Level:.8.) The wider the interval for a given sample size, the more confident we are of the interval reaching the population mean. Topic 33 used the same data but used a known s and obtained an 80 percent confidence interval of to for a width of 2.02 inches ( = 2.02 inches). The interval above is wider for two reasons: the critical t is bigger than the critical z (1.833 compared to 1.28) and Sx is bigger than s (2.72 compared to 2.50). The Sx will change from sample to sample, so it is possible that the interval width could sometimes be smaller than the s case. If we consider all the possible variable width intervals, 80 percent will contain the true population mean. Using the Equation Solver for Critical t Values For a 90 percent confidence interval, there is an area of (1-0.90)/2 = 0.05 in each tail. We want to solve tcdf(x, â99, 9) = 0.05 for X, the X that gives the area of 0.05 in the right tail (from X to â99) of a t distribution with nine degrees of freedom (n - 1 = 10-1 = 9). 1. Press 0:Solver for screen 25. If your screen does not start with EQUATION SOLVER, you must first press }. 2. Enter your equation using y [DISTR] 5:tcdf( X â99 9 ¹ 0 Ë Press Í, and modify the screen to look like screen 26. We pick X = 2 for t as a reasonable first guess in the right tail. The bound will probably show as {-1å99, 1å99}, which works fine; although you could restrict it to positive values by making it {0, 1å99}. 4. With the cursor blinking on or after the 2, press ƒ [SOLVE]. Notice the busy signal while the equation is being solved numerically. Then screen 27 appears showing a critical t value of (24) (25) Note: We write the equation to be solved as 0 = tcdf(x, â99, 9) to meet the requirements of the Solver. (26) (27) 1997 TEXAS INSTRUMENTS INCORPORATED STATISTICS HANDBOOK FOR THE TI-83 77

6 Activity 7, Estimations and Confidence Intervals (cont.) Topic 35 (42) Estimating a Population Proportion A random poll of 265 people from a population of interest found 73.2 percent agreement with a public policy. What is the 95 percent confidence interval for the proportion in the whole population who would agree? (Note that and 265(1-.732) 10, so a normal distribution can be used to approximate a binomial.) 1. Press <TESTS> A:1-PropZInt for screen 28. The input screen is looking for the number who agreed (x), and this was not given in this particular problem. 2. Let the TI-83 do the calculation for you ( ). However, when you leave this line, the calculation reveals a noninteger, (see screen 29). 3. Highlight Calculate in the last row, and press Í. A DOMAIN error results. 4. As shown in screen 30, round the value to the nearest integer (194), highlight Calculate in the last row, and press Í for screen 31. We are 95 percent confident that between to percent of the population agree with the public policy. Home Screen Calculations The margin of error is =.0533 as shown in screen 32. The confidence interval is calculated in screen 33. (28) (29) (30) (31) (32) (33) 78 STATISTICS HANDBOOK FOR THE TI TEXAS INSTRUMENTS INCORPORATED

7 Finding a Sample Size What size sample would be needed to reduce the margin of error in the above problem from to 0.02, or 2 percent? Solving for n in our margin of error equation, we have n = ä 0.732( )/ = 1885 (rounding up). (See screen 34.) The original sample was generated (as described in Topic 25 and shown in screen 35) from a population with a population proportion of Thus the interval obtained (67.88 to percent) does contain the 70 percent. (See screen 35.) For the larger sample (n = 1885) shown in screen 36, the resulting is within 0.02 of (34) (35) (36) 1997 TEXAS INSTRUMENTS INCORPORATED STATISTICS HANDBOOK FOR THE TI-83 79

8 Activity 7, Estimations and Confidence Intervals (cont.) Topic 36 (43) Estimating a Normal Population Standard Deviation σ A random sample of size 4 from a population that is normally distributed is as follows. {54.54, 43.44, 54.11, 46.88} = L1. Find the 90 percent confidence interval for the population standard deviation. The confidence interval is based on the sample variance and the c 2 distribution (the similarity between the distribution of sample variance and the c 2 distribution can be seen in Topic 39). A. Variance = With the data in L1, calculate the variance of the data by pressing y [LIST] <MATH>8:variance( L1 Í for as in screen 37. With n = 4 pieces of data, we will be using n - 1 = 4-1 = 3 degrees of freedom for the c 2 distribution. The plot is given in screen 40 (with setup as in screens 38 and 39). As you can see, the plot is skewed to the right. Because it is not symmetrical, two critical values are needed, c 2 L and c 2 R, for left and right. (Shading was done with y [DRAW] 7:Shade( 0 Y1 0 0 Ë 35 and Shade( 0 Y1 7 Ë 8 â99.) Thus, 5 percent of each tail is shaded. See statement 2.) B. Critical values are c 2 L = and c 2 R = You can look up the critical values in a table (with 0.05 in each tail of the distribution), or you can obtain them using the equation solver as explained at the end of this topic. C. Confidence intervals for variance = 11.52, Lower value = (n - 1)Sx 2 /c 2 R = 3 ä 30.00/7.814 = Upper value = (n - 1)Sx 2 /c 2 L = 3 ä 30.00/0.352 = D. Confidence interval for standard deviation = 3.39, Lower value = ((n - 1)Sx 2 /c 2 R) = (11.52) = 3.39 Upper value = ((n - 1)Sx 2 /c 2 L) = (255.68) = The above sample was the first of 100 randomly generated samples in Topic 39 from a normal distribution with a variance σ 2 = 100 and a standard deviation σ = 10. These, in fact, do lie in the intervals calculated. The intervals are quite wide, but they are based on a very small sample. (37) (38) (39) (40) (41) 80 STATISTICS HANDBOOK FOR THE TI TEXAS INSTRUMENTS INCORPORATED

9 Using the Equation Solver for Critical Values c 2 L and c 2 R We want to solve the following equations for X such that the area (from 0 to X and from X to the very large number â99) is 0.05 under a c 2 distribution with n - 1 = 4-1 = 3 degrees of freedom. (i) c 2 cdf(0, X, 3) = 0.05 (ii) c 2 cdf(x, â99, 3) = 0.05 For c 2 L: 1. Press 0:Solver for screen 42. If your screen does not start with EQUATION SOLVER, you must first press }. 2. To meet the requirements of the solver, write equation (i) as in screen 42 using y [DISTR] 7: c 2 cdf(. 3. Press Í and modify the screens to look like screen 43. We pick X = 1 for our first guess for the left tail. The bounds will probably show as {-1å99, 1å99}, which works fine, although we could restrict them to positive values {0, 1å99} because c 2 cannot be negative. 4. With the cursor blinking on or after X = 1, press ƒ [SOLVE]. The critical value X = = c 2 L is calculated. See screen 44. (During calculations, a busy signal shows in the upper-right corner of the screen.) For c 2 R: Repeat the above procedure for the second equation (ii) as in screen 45. Use a first guess of 10, as in screen 46, for the critical value c 2 R = as in screen 47. (42) (43) (44) (45) (46) (47) 1997 TEXAS INSTRUMENTS INCORPORATED STATISTICS HANDBOOK FOR THE TI-83 81

10 Activity 7, Estimations and Confidence Intervals (cont.) Topic 37 (44) Estimating the Difference in Two Population Means Estimating Independent Samples A study designed to estimate the difference in the mean test scores that result from using two different teaching methods obtained the following data from two random samples of students taught with the two methods. Method A (L1) Method B (L2) mean Sx n Store the above data in L1 and L2, and calculate the 95 percent confidence interval for the difference in the population means. TInterval (σ1 and σ2 Unknown) Assume the above data is from normal populations. 1. Press <TESTS> 0:2-SampTInt for screens 48 and 50 (48\$ and 50\$ are the second page of the input screens obtained by using ). Screens 48 and 50 do not pool the sample variances as does screen 52. (For the pooled case, see To Pool or Not to Pool on the next page.) Screen 48 uses the Data List. However, for screen 50, you must enter the summary Stats of means, standard deviation, and sample sizes. 2. Make the entries as shown in screen 48 or 50, highlight Calculate, and press Í for screens 49 or 51. The 95 percent confidence interval for the difference of the means is to for the non-pooled case. Because zero is in the interval, there could be no difference in the mean test scores for the two groups, or Method A could have a higher mean than Method B (a positive difference), or Method B could have a higher mean than Method A (a negative difference). Perhaps larger samples would clarify the situation. (48 48\$) (49 49\$) (50 50\$) (51 51\$) 82 STATISTICS HANDBOOK FOR THE TI TEXAS INSTRUMENTS INCORPORATED

11 To Pool or Not to Pool Screens 52 and 53 give the input and output for the pooled procedure with a 95 percent confidence interval of to The pooled procedure is appropriate if s1 = s2. It is not appropriate to pool if s1 ƒ s2. Some statistics textbooks only give the pooled version because before the TI-83, the noninteger value for degrees of freedom that results from the non-pooled procedure was difficult to handle without a computer. If you are not sure that s1 = s2, then it is safe to use the nonpooled procedure because it will always give a conservative answer. (The confidence interval will be a little wider than in the pooled case (-.839 to compared to to )) ZInterval (σ1 and σ2 Known) Assume the above data is from normal populations but that s1 = and s2 = Press <TESTS> 9:2-SampZInt for screen 54. The Data Lists input is also possible, but only the Stats version is shown. Some texts use this procedure for large samples when s1 and s2 can be approximated by Sx1 and Sx2. 2. Make the entries as shown in screen 54, highlight Calculate, and then press Í for the output shown in screen 55. The 95 percent confidence interval for the difference of the means is to This interval is similar to, but not as wide as, the Tintervals above. (52 52\$) (53 53\$) (54 54\$) (55) 1997 TEXAS INSTRUMENTS INCORPORATED STATISTICS HANDBOOK FOR THE TI-83 83

12 Activity 7, Estimations and Confidence Intervals (cont.) Home Screen Calculations Screens 56, 57, and 58 show the non-pooled TInterval. First, the degrees of freedom (8.134) are calculated; second, with (1 -.95)/2 = in each tail, the critical t value of is checked. (The critical t value is calculated by using the equation solver to solve tcdf(x, å99, 8.134) = for X = 8.134, as explained in Home Screen Calculations in Topic 34.) Next, the margin of error and confidence interval are calculated. Screens 59, 60, and 61 are for the pooled case with degrees of freedom = n1 + n2-2 = = 9 and a critical value of (from a table or using the procedure explained in Topic 34). Screens 62 and 63 are for the ZInterval. Note that the calculation for the ZInterval confidence interval is the same as for the non-pooled TInterval except for the differences in the critical z and t values. (56) (57) (58) (59) (60) (61) (62) (63) 84 STATISTICS HANDBOOK FOR THE TI TEXAS INSTRUMENTS INCORPORATED

13 Estimating Dependent Samples To test a blood pressure medication, the diastolic blood pressure readings of a random sample of ten people with high blood pressure were recorded. After a few weeks on the medication, their pressures were recorded again. See the data recorded in the table below. The mean and the standard deviation of the differences in L3 were calculated with 1-Var Stats as explained in Topic 4 and shown in screens 64 and 65. Note that the mean of the differences is 5.9 and the standard deviation of the differences is (64) (65) Subject Before (L1): After (L2): L1 - L2 = L Assume the diastolic blood pressure readings are normally distributed, and calculate the 95 percent confidence interval for the mean difference in pressure. 1. Press <TESTS> 8:TInterval for one of the two possible screens (see screen 66 or 68), depending on what input (Inpt:) is highlighted. 2. Put in the correct values as above, highlight Calculate in the last row, and then press Í for the output in screens 67 or 69. We are 95 percent confident that the difference in the population mean lies between 1.85 and 9.95 units of pressure, and this indicates that the medication seems to have some effect. Home screen calculations are similar to those done in Topic 34. (66) (67) (68) (69) 1997 TEXAS INSTRUMENTS INCORPORATED STATISTICS HANDBOOK FOR THE TI-83 85

14 Activity 7, Estimations and Confidence Intervals (cont.) Topic 38 (45) Estimating the Difference in Two Population Proportions A polling organization found 694 of 936 (694/936 = percent) women sampled agreed with a public policy while 645 of 941 (68.54 percent) men agreed. Find the 95 percent confidence interval for the difference in the proportions between the two populations sampled. Women: x1 = 694 n1 = 936 Men: x2 = 645 n2 = Press <TESTS> B:2-PropZInt for screen Enter the correct values (as in screen 70), highlight Calculate in the last row, and then press Í for the output in screen 71. We are 95 percent confident that the difference in population proportions lies between 1.52 and 9.68 percent. This indicates that a larger proportion of women are in agreement with the policy than are men. Home screen calculations are given in screens 72 and 73 with the critical z value of 1.96 and a margin of error of 4.08 percent. These calculations also could have been obtained from screen 71 by taking ( ) = The confidence interval from 1.53 to 9.69 percent differs slightly from the above because of rounding. Note: n1 = 936 = (both values greater than 5). n2 = 941 = (both values greater than 5). Therefore, the normal approximation to the binomial can be used. (70) (71) (72) (73) 86 STATISTICS HANDBOOK FOR THE TI TEXAS INSTRUMENTS INCORPORATED

15 Topic 39 Unbiased and Biased Estimators (Simulation) For this topic, set the mode for two decimal places. (Refer to Setting Modes under Do This First.) A sample statistic used to estimate a population parameter is unbiased if the mean of the sampling distribution of the statistic is equal to the true value of the parameter being estimated. You saw in Topic 24 that the sample proportion is an unbiased estimator of the population proportion. In Topic 25, you saw that the sample mean is an unbiased estimator of the population mean. In this topic, you will see that Σ(x - þ) 2 /n is a biased estimator of the population variance while Σ(x -þ) 2 /(n -1) is unbiased. Because Sx 2, the variance on the TI-83, is unbiased (divides by n - 1), you will need to change to biased (maximum likelihood estimator) by multiplying by (n - 1)/n as ((n - 1)/n) Σ(x - þ) 2 /(n - 1) = Σ(x - þ) 2 /n. For n = 4, you would multiply by 3/4 or After making the above change, proceed as follows. 1. Set the random seed as explained in Topic 21 and shown in the first two lines in screen Press <PRB>6:randNorm( Í to simulate picking four values from a normal distribution with a mean of 50 and a standard deviation of 10 or a variance of 10 2 = 100. The values are 54.54, 43.44, 54.11, and (middle values in screen 74). 3. Press y [LIST] <MATH> 8:variance( y [ANS] Í for 29.99, an unbiased variance of these values (last value in screen 74). 4. Reset the seed and generate a sequence of 100 sample variances. Store these in L1, as in screen 75, with seq pasted from y [LIST] <OPS> 5. Notice that the first variance is as above. (74) (75) (76) 5. Obtain the mean of L1 by pressing y [LIST] <MATH> 3:mean( L1 Í, as in screen 76. The is very close to the population value of Set up Plot1 for a Histogram of the data in L1 (as explained in Topic 1) with the WINDOW shown in screen 77 for the results in screen 78. (77) (78) 1997 TEXAS INSTRUMENTS INCORPORATED STATISTICS HANDBOOK FOR THE TI-83 87

16 Activity 7, Estimations and Confidence Intervals (cont.) For the biased estimator, you can multiply each of the unbiased calculations for the variance by 0.75 as explained earlier and as shown in screen 79. These have a mean of 74.49, which is not near the population value of 100. The remaining screens (80, 81, and 82) give the procedure and results if you had started from scratch in calculating the biased estimator with the mean and a Histogram of your sample distribution. Note that the sample distribution is skewed to the right. This is not surprising when you realize that the left is bounded by zero because the variance is a sum of squares and, thus, can never be negative. The c 2 distribution is related to this distribution and is discussed in Topic 36. (79) (80) (81) (82) 88 STATISTICS HANDBOOK FOR THE TI TEXAS INSTRUMENTS INCORPORATED

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