Question

Excel' has functions for finding x-values or z-values associated with a cumulative probability from a normal distribution. This cumulative probability must be an upper-tail - more than - probability.

Answer 1

P(Z < z ) = p

to calculate z

=normsinv(p)

for example for p = 0.95

z = normsinv(0.95)

= 1.645

however you want z-critical value for confidence interval of 100 (1-p) %

then you have to write

normsinv(1 -p/2)

for example for 95% confidence

=normsinv(0.975)