A random sample of 75 charges on a credit card had a mean of $56.85, and the population standard deviation is

$21.08. Compute an upper 95% confidence bound for the mean amount charged.

A confidence interval provides likely minimum and maximum values for a parameter.
In some cases, we are interested only in a maximum or only in a minimum. In these cases, we construct a one-sided confidence interval. A one-sided confidence interval can be an upper confidence bound, which has the form x̄ + zα σ∕√n, or a lower confidence bound, which has the form x̄ − zα σ∕√n. Note that the critical value is zα rather than zα∕2.