Use the given confidence level and sample data to find a confidence interval for the population standard deviation. Assume that a simple random sample has been selected from a population that has a normal distribution.

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80% confidence; n= 91, x = $66,400, s= $ 19,811

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  1. Look in the back of your statistics textbook for a table called something like “area under normal distribution” to find the proportion/probability ± .10, and use its Z score.

    ± Z = (score-mean)/(SD/√n)

    Calculate for the Z score both positive and negative.

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