t time When constructing confidence intervals for a population mean, we almost always use critical values from a t distribution rather than the standard Normal distribution.

(a) When is it necessary to use a t critical value rather than a z critical value when constructing a confidence interval for a population mean?

(b) Describe two ways that the t distributions are different from the standard Normal distribution.

(c) Explain what happens to the t distributions as the degrees of freedom increase.