Question

An Airline executive is interested in knowing the average travel time of all fights from LAX to Chicago. She takes a random sample of 50 flights from LAX to Chicago.
The sample mean ( x bar ) is 180 mins. Th standard Deviation of travel of all flights from LAX to Chicago ( Sigma ) is known to be 20 mins . What is the formula to find 95% confidence interval ? ( Excel Format. )

Answer 1

Sample size,n= 50

Sample mean ,  

Popoulation standard deviation ,

For 95% confidence interval , z=1.960

Confidence interval for population mean is

  

So   

Thus confidence interval is :

In excel format formula for margin of error is :

= CONFIDENCE (alpha, standard_deviation, size)

Where alpha= 1- confidence level = 1- 0.95= 0.05

So here formula is

= CONFIDENCE (0.05,20,50)

Output : 5.1437

Now in the next cell for lower bound use

= Mean - the value obtain from upper formula

= 180 - 5.1437 = 174.8563

For upper bound of confidence interval use :

= Mean + 5.1437

= 180 + 5.1437= 185.1437