The batteries produced in a manufacturing plant have a mean time to failure of 30 months, with a standard deviation of 2 months. I select a simple random sample of 400 batteries produced in the manufacturing plant. I test each and record how long it takes for each battery to fail. I then compute that the average failure time of the 400 batteries is 29.9 months with a standard deviation of 2.15 months. In this scenario, the value 30 is: