90% CONFIDENCE FOR THE MEAN TIME TO COMPLETE A TEST. POPULATION STANDARD DEVIATION OF 3 HOURS. SAMPLE SIZE IF THE ERROR SHOULD BE LESS THAN A HALF HOUR?

Answer 1

90% = mean ± Z (SEm)

Look in the back of your statistics textbook for a table called something like “area under normal distribution” to find the proportion/probability (± .05) and its Z score.

90% = mean ± 1.645 SEm

.5 = 1.645 SEm

SEm = SD/√n

Solve for n.