The t Distribution (1)• It is the usual case, in situations such as have beenpresented, that the population variance, as well as thepopulation mean, is unknown. This condition presents aproblem with respect to constructing confidence intervals.Although, for example, the statisticis normally distributed when the population is normallydistributed and is at least approximately normallydistributed when n is large, regardless of the functionalform of the population, we cannot make use of this factbecause is unknown.