Unformatted text preview: l Rule, the margin of error for a 95% confidence interval is 2 standard errors. The number of standard errors contained in the margin of error is called a critical value. A critical value is a Z
score, denoted !! , that corresponds to a certain level of confidence. While the Empirical Rule provides approximate critical values, exact critical values can only be found using the normal distribution. 8 To find the critical value for a 90% confidence interval, we must find the Z
scores, – Z and Z, such that the middle 90% of the standard normal distribution is between – Z and Z. Below is a plot of the standard normal distribution (mean = 0, standard deviation = 1). Find the value of the Z
score that has 5% area to its left, and the value of the Z
score that has 5% area to its right. 5% 4 9 90% 2 Z 0 5% Z 2 4 What do you notice about these Z
scores? What explains this relationship? 10 What percent of a normal distribution will be within 1....
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 Spring '14

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