Summary
Students sometimes have difficulty deciding whether to use z sub alpha/two or t sub alpha/two values when finding confidence intervals for the mean. As stated previously, when the population standard deviation is known, z sub alpha/two can be used no matter what the sample size is, as long as the variable is normally distributed or n≥30. When population standard deviation is known and n≥30, s can be used in the formula and z sub alpha/two values can be used. Finally, when population standard deviation is unknown, and n<30, s is used in the formula and t sub alpha/two values are used, as long as the variable is approximately normally distributed.




V.E. Sabusap