99% confidence interval is 2.575, why?

When asked to determine the range of the monthly returns that fall within 99% confidence interval I looked in the Z table an found .9901 is equivalent to 2.33

However, I got the wrong answer as the right answer used Z statistic reliability factor of 2.575 (That is .9949 in the Z table)



You put the entire 1% (= 1 – 99%) into the right tail (as if you were constructing a one-tail hypothesis test). Their interval put 0.5% in the right tail and 0.5% in the left tail, which is the common way to construct a confidence interval.