Why we subtract the value of a(alfa)/2 from 1?In Z(a/2) and what is the purpose of reliability factor?
The reason is because the z-table is provided in this way. As you will see a couple of lectures later, the T-table is provided in a different way. Moreover, later on we will exclusively use p-values to find these values.
Regarding the reliability factor – it is a main ingredient in the calculation of confidence intervals. It helps us determine exactly how wide the CI will be.