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7.5 Estimation of the Population Standard Deviation Most populations are so large; the majority of studies have little to no information about the population variance deviation . 2 or standard Why do we need to estimate standard deviation? - GE Company manufactures cat-scan machines with an automatic injection of iodine to blood stream. The consistency of the amount of iodine injected each time is subject to strict safety measurements. - Bolts used in constructing air planes undergo a tremendous amount of abuse (weather, pressure, friction etc.). How can a manufacturer guarantee that each bolt they manufacture will consistently function properly under such conditions? When a sample is drawn from a normally distributed parent population, the quantity (n 1) s2 2 is known to follow a Chi-Square distribution with n-1 degrees of freedom, denoted as 2n1 ( the proof of this fact is beyond the scope of this course) The Chi-Square Distribution Is a right skewed distribution with mean n-1 and a variance of 2(n-1). The 2n1 distribution is a family of distributions determined by the sample size (Table 6 page 832). Let’s Do It! 1 Determine the mean and standard deviation of 322 . Confidence Interval for the Population Variance 2 A (1 )% confidence interval for variance 2 of a normally distributed population is given by: (n 1)s 2 2 right 2 (n 1)s 2 2 left Confidence Interval for the Population Standard Deviation A (1 )% confidence interval for the standard deviation of a normally distributed population is given by: (n 1)s 2 2 right (n 1)s 2 2 left Let’s Do It! 2 Use textbook Table G, to find the following chi-square values .299,12 Example .201,12 Find the 95% confidence interval for the variance and standard deviation of the nicotine content of cigarettes manufactured if a sample of 20 cigarettes has a standard deviation of 1.6 milligrams. Solution Since 0.05, the two critical values, respectively, for the 0.025 and 0.975 levels for 19 degrees of freedom are 32.852 and 8.907. The 95% confidence interval for the variance is found by substituting in the formula. Hence, one can be 95% confident that the true variance for the nicotine content is between 1.5 and 5.5. For the standard deviation, the confidence interval is Hence, one can be 95% confident that the true standard deviation for the nicotine content of all cigarettes manufactured is between 1.2 and 2.3 milligrams based on a sample of 20 cigarettes. Let’s Do It! 3 A sample of 20 cigarettes of a certain brand has a standard deviation of 1.6mg. Estimate the standard deviation of nicotine content of this brand at a 90% confidence level. Assume normality of the nicotine content in cigarettes. Estimate the variance of nicotine content of this brand at a 90% confidence level. Let’s Do It! 4 Find the 90% confidence interval for the standard deviation for the price in dollars of an adult single-day ski lift ticket. The data represent a selected sample of nationwide ski resorts. Assume the variable is normally distributed. 59 54 53 52 51 39 49 46 49 48 Interpret the interval Homework 7.5 Page 382: 3-12 all