The assumptions are the same for using T or Z in constructing a confidence interval for the population mean.
Ideally, a random sample is taken
• The population of measurements is normally distributed or a sample of at least
30 is taken.

When do we use a t distribution for estimating the population mean with a
confidence interval?

A. The confidence interval is less than 90%
B. The sample mean is very small.
C. The confidence level is close to 100%
D. The population standard deviation is unknown.
E. The sample mean is very large.