T-Value

In sampling with a normal distribution, a factor called the z-value is used for determining confidence intervals around a sample mean. The z-value is used when the data in the population are normally distributed with a known standard deviation. When the data in the population are not normally distributed or the standard deviation of the population is not known, the appropriate factor to use for the same purpose is called a t-value. It is based on the t-distribution, which approximates the normal distribution, especially as the sample size increases. See also regression analysis and coefficient of correlation and coefficient of determination and standard error of the estimate.