To test for an x distribution that is mound-shaped using sample size n 30, how do you decide whether to use the normal or Student's t distribution? If is known, use the standard normal distribution. If is unknown, use the Student's t distribution with n - 1 degrees of freedom.For large samples we always the standard normal distribution. If is unknown, use the standard normal distribution. If is known, use the Student's t distribution with n - 1 degrees of freedom.If is known, use the standard normal distribution. If is unknown, use the Student's t distribution with n degrees of freedom.