The mass of a particular type of steel sheet produced by a factory has a normal distribution. The mean mass of a random sample of 14 sheets is 72.5kg and the standard deviation is 3.2kg. Calculate a 95% confidence interval for the mean mass of the steel sheets produced by the factory.

I just want to ask:

The mean mass of a random sample of 14 sheets is 72.5kg and the standard deviation is 3.2kg.

Is this 3.2kg a sample standard deviation?

Should i do like this?

or:

this (3.2kg)^2 is already an unbiased estimate of population variance?