The following data represent the muzzle velocity (in feet per second) of shells fired from a 155-mm gun. For each shell, two measurements of the velocity were recorded using two different measuring devices, resulting in the following data.

Observation 1 2 3 4 5 6
A 792.5 793.8 792.5 791.9 792.2 791.1
B 795.8 797.1 793.6 797.3 789.9 797.9
(a) Construct a 99% confidence interval about the population mean difference. Compute the difference as device A minus device B. Interpret your results.