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T11-01 - 1 T11-01 1 Population Variance Confidence Intervals Purpose Allows the analyst to analyze the population confidence interval for the variance and standard deviation based on a 1- confidence level. Inputs Confidence level Sample variance (Std Dev) Sample size (n) Outputs Confidence interval (LCL, UCL) for variance Confidence interval (LCL, UCL) for standard deviation

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T11-01 - 2 Confidence Interval for Variance Lower Confidence Limit (LCL) Upper Confidence Limit (UCL)

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T11-01 - 3 An Example All spark plugs in a car are usually changed at the same time. A spark plug manufacturer has been working on an improved (less variation in plug life measured in miles driven) plug. A random sample of 20 of the newly developed plugs were taken to determine the variability in plug life. Develop a 99% confidence interval for the variance and standard deviation of the new spark plugs if the sample produced a sample variance result of 800 plug life miles.

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T11-01 - 4 Input the confidence level (.XX for XX%), Sample variance (s^2), and n. The confidence interval for the variance and standard deviation is automatically calculated

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T11-01 - 5

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