Confidence interval using a t-value
The interval estimate
has a lower confidence level than 95%. In order to achieve a 95% confidence level, the interval must be widened. This is done by replacing 1.96 by a slightly larger number:
The replacement constant tn-1 is a value greater than 1.96 that depends on the sample size, n. The value n - 1 is called the degrees of freedom of the constant.
For any number of degrees of freedom, the t-value can be obtained from a table, graph or computer software.