Assuming the standard deviation is unchanged and that the usage is normally distributed, provide an expression for calculating a 99% confidence interval for the mean usage in the March quarter of 2006. (Exactly one option must be correct)
Since "\u03c3" is assumed known,
we use the interval "x\u0304 \u00b1 z \\dfrac{\u03c3}{ n},"
where "\u03c3 = 81, n = 30" and where z is chosen to ensure that "P(|Z|\u2264 z) = 0.99."
From the normal tables, "P(|Z|\u2264 2.575) = 0.99" (because "P(Z < 2.575) = 0.995)" and so we use "z = 2.575."
99% confidence interval expression "= \\bar{x}\\pm 2.575\\times \\dfrac{\\sigma}{n}"
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