Step 1 (a) If we want to estimate the population mean time for previews at movie theaters with a margin of error of 75 seconds, what sample size should be used? Assume 95% confidence. Recall the formula to find a sample size, n, for a desired margin of error, E, with a population standard deviation, and z relates to the level of confidence. a/2 (2/2)202 n = The sample standard deviation of 5 minutes will be used in place of the population standard deviation. However, note that the desired margin of error was given to be 75 seconds. The standard deviation and margin of error must be in terms of the same units. Convert 75 seconds to minutes. 75 seconds. 1 minute minutes 60 seconds Therefore, the margin of error, in minutes, is E = minutes.