## Question 685:

Asked on April 18, 2009

Tags:
Sample Size ,
Margin of Error

1

## Answer:

No answer provided yet.We need to estimate the sample size given a margin of error of 1.5 hours and level of confidence of 95%. The procedure to do this involves working backwards from the procedure used to generate a confidence interval. Recall that the confidence interval is 2 times the margin of error. So we need to work backwards from the margin of error of 1.5 to find the sample size.

- First, we need to find the critical value for a 95% level of confidence, which we obtain from a table of normal values or the percentile to z-score calculator and we get 1.9599.
- Next, we divide the desired margin of error by the critical value = 1.5/1.9599 = .76532
- Divide the standard deviation by the result from step 2 = 6.4/.76532 = 8.362513
- Finally, we square the result from Step 3 = 69.93162.

Rounding up to the nearest student we should plan on 70 students.