How large a sample should the civil engineer take to be 90% confident of his estimate?

Statistics
Tutor: None Selected Time limit: 1 Day

A civil engineer wishes to estimate the average completion time of freeway widening projects correct to within 1 month. From previous experiments it is known that the population standard deviation is approximately 6 months. How large a sample should the civil engineer take to be 90% confident of his estimate?

Dec 18th, 2014

Tabular value for 90% is 1.645

1.645 * sigma /sqrt(n) = 1 month

1.645 * 6/1 = sqrt(n)

sqrt(n) = 9.87

n =9.87^2=97.42 = 98

Answer: n >= 98

Dec 18th, 2014

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