StatisticsdrillMargin of error for a mean (z, σ known)
A statistics problem from Statisticsdrill. Solve it with one answer box, with a fresh sample on every attempt and the worked steps and spreadsheet function in the answer, and it is checked the moment you submit.
The task
The population standard deviation of call durations is known to be σ = 2 seconds. A random sample of 25 is taken. What is the margin of error of a 99% confidence interval for the population mean? Use z* = 2.576. Round to 2 decimal places.
What this problem teaches
The formula: Margin of error = z* × σ / √n. Excel uses the unrounded z*, so its result can differ from this one in the third decimal.
A hint if you get stuck
With σ known the interval is x̄ ± z* × σ / √n. The margin of error is the part after the ±.
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