# Why standard deviation of population is unknown in t test

Why mean of the population is known but standard deviation is unknown ? Mean is calculated from individual values then why can't standard deviation be calculated? Is there any practical scenario where mean is known but standard deviation is unknown?

• It is a test about the mean: one specific value of the mean is tested as being compatible (or not) with the sample. The true mean is unknown, like the standard deviation. – Xi'an Nov 26 '15 at 17:35